More on using the definition of game to help plan games:
1. Some goal or goals to be met (i.e., victory conditions)
It behooves the GM to consider what goals the PCs might meet, because meaningful PC activity is always related to some form of goal. Goals allow the PCs to engage with the world. In its simplest form, a goal is something that the PCs (and players) care about, from "acquire loot" to "protect the town" to "get even with the rat who set us up".
When an actor says, "What is my motivation?" he is asking what the goal of his character is in a given scene. Similarly, failure to provide clear goals prevents players from acting in an appropriate way within the game. If your players don't know what to do, reconsider how you set goals.
When creating a plot, it is sometimes easiest to start with the NPC goal, and then determine why the PCs would care. Why would they want to foil the NPC? What is their goal? How would they define victory?
And "victory conditions" are important for satisfactory resolution. The players should be able to tell when they've scored against their foes, when they have been victorious, and when they have only partially met their goals. By thinking ahead of time about what is required to change the course of mighty rivers (so to speak), the GM can not only supply the information required to help the players set appropriate sub-goals (i.e., the steps needed to meet the bigger goal) but can also ensure that when the mighty rivers' courses move, there is a real impact on the campaign milieu.
When this happens, victory seems real. When it does not happen, victory seems hollow.
2. Player actions/choices related to said goal or goals
Once you know what is likely to motivate your players (and their characters), it is possible to consider the actions/choices they are likely to make which relate to those motivations.
If the GM expects the PCs to examine six adventuring locations on the way to achieving the great goal, the GM better make sure that each of those locations relate to the overall goal in some way. They can provide equipment, information, or passage to another location, but they must provide something that motivates the players, and allows the players to view progressing through them to be part of the overarching goal.
The players must have a sense of their own progress. They must believe that they can affect the outcome, or they will not be motivated to try.
3. The resolution of said goal or goals is unknown when the game begins
Similarly, the players must believe that there is something at stake. If they are playing an "avert the end of the world scenario" knowing ahead of time that the world will not end, this knowledge not only robs their efforts of any sense of urgency, but it also robs their choices of any meaning.
In a role-playing game, the resolution of the goal(s) is unknown due to the opposition (both direct opposition and incidental opponents, such as wandering monsters) the PCs experience. The PCs must work to overcome enemies, avoid pitfalls, garner allies, and gain the information/gear they need to meet their goal. While any single failure or success doesn't mean the success or failure of the whole enterprise, each makes overall success more or less likely. It is the tension of unknown resolution which makes players sit at the edges of their seats.
If the outcome of some aspect of the session is known, consider it part of the framework, and move past the framework into the unknown as fast as you can. If the outcome of some aspect of the session is unknown,
and is important to the PCs' goals, consider it part of the game, and allow the players to engage with it as much as they desire. At no time should the players be suckered into spending large chunks of time on anything that will not change no matter what their PCs do.
For example, if you are playing A1-A4 in order, don't bother with the fight at the end of A3. It's part of the framework. It isn't part of the game. Narrate what happens, and get on with A4. In your own creations, try to make the framework as light and as permeable as possible. Most players are at the table to make meaningful choices, not to listen to long narratives.
If your players are bored, consider the following:
1. Have I mistaken framework for game? IOW, am I wasting too much precious game time on things that either do not affect PC goals, or that the PCs cannot affect?
2. Do the PCs know the victory conditions? IOW, do the PCs know enough to have clear goals, or are they wandering aimlessly? Even in a sandbox, it is possible to set clear goals for each session, as well as clear goals for overarching action. If you read the advice Gary Gygax gave to players in the 1e PHB, you may note that he recommends setting clear goals for each session, even if it's only "find the entrance to the next level".
It's okay to have the victory conditions change in mid-session. It is not okay to go long stretches of game time without any possibility of setting a goal.
3. Have I given the PCs some clear idea of actions that can affect goals? IOW, re-examine the hooks you are using, and make sure that they include not only the goal to be met, but some idea of ways to take action to meet them.
On a related note, try to avoid time wasters that don't have anything to do with player goals. If you play a game with serious combat grind, consider how much time is wasted to slay a few kobolds that have little to do with the PC's goals.
It is not only true that the players should be able to take meaningful action toward their goals, it is true that the resolution of events should take game time which is at least vaguely proportional to the amount said events are important to those goals. Half an hour to an hour spent on a combat with wandering monsters is just too bloody long if you are going to have a lot of wandering monsters.
4. Do the players already know how this will end? Do they believe they do? If there is no doubt how the goals will resolve, then there is no tension.
Throw something else into the mix.
Or accept that the remainder of the "adventure" has already become framework. The outcome is known. Narrate it briefly, and move on to the next thing.
RC