Some or all of my answers will be less mechanical and more philosophical.
1) Rewards. Published adventures rarely use XP anymore - just milestones. Which is okay, but what do you use to motivate your players? Treasure? It's awarded randomly, inconsistently across adventures. Magic items don't really matter anymore, yet players want them. And if you award them, the already straining encounter building system gets more fragile.
An intriguing mystery to solve with interesting encounters all along the way is its own reward. The game has moved into adventure based motivation. Thats a change up that old school players and those perhaps more familiar with video games may struggle with. My suggestions are either as GM try and make the game more interesting in exploration and social pillar, or use a system that built to play more old school skill play dungeon crawler.
The longer answer is 5E would have added a module to the game to allow folks to apply an old school reward system. Magic item economy, more experience point depth, etc... 5E ended being too popular to need any modularity leaving folks to figure it out themselves. Im not excusing it, im just explaining it. This isnt just a you and your group problem either.
2) Encounters. Players streamroll encounters. Or it's a TPK. Neither is fun. I want a dynamic experience where everyone is leaning over the table, cheering over a critical hit, high-fiving another player whose Bless spell made it happen. I understand that this can't be every die roll - or even every encounter or even every session - but to have no energy or enthusiasm for weeks/months on end is disheartening.
In my experience the place and the stakes of the encounter can help here. If the encounters are straight up "enter room; roll initiative" then its entirely on the mechanics of those fights to be interesting. 5E harkens back to a more strategic style of play, where the interesting bits happen before battle. Where are you fighting? Did you bring the right spells and weapons? Do you have a viable exit? Its not a tactical piece that makes combats themselves an interesting puzzle.
Longer answer, there is a lot of shenanigans involved in short and daily rests existing in the same system. Advice says 6-8 encounters a day for a balanced party. That could be spot on, but I think its off for many folks. The CR is a guideline because combats are a bit less predictable than folks would like. PF2 has a solid and predictable encounter guideline for tactical combats. Some folks might prefer a less fast and furious, yet team dependent experience.
3) Attention. Care about the story/plot/NPCs - at least a little. Get scared when you encounter Strahd. Feel a little solemn when the beloved NPC sacrifices his life for you. Just ... care a little, you know? [I think some of this is actually connected to the first two points. There's just no stakes, nothing to care about, when the game is easy and doesn't have rewards.]
Do you think the players would care more if they knew Strahd was worth X amount of XP? Would they be more concerned about the NPC if keeping them alive gave them magic item rewards? My experience says no, its entirely why I stopped using XP. Organic play is only going to happen between you and the players. You cannot carrot and stick them into it. My advice would be to give them more RP time with NPCs. Take note of any interaction or interest of the players. Feed into that. Give them time to discover the world and the situations so they can react accordingly. Or, they might nor care and just want beer and pretzel dungeon delving. Takes time to figure out one way or the other.
4) Learn the system. I've run for a variety of 5e groups. Many of the players aren't motivated to learn the rules or their characters. Some of this is that the system isn't (IMO) designed to be especially intuitive.
What system is? A serious question. Most RPGs are complex and nuanced, and whats worse, is they compete with video games that off load all that cognitive load. In my GM experience, you have to be ready to lead and sometimes carry that part of the game for certain players. Especially, younger ones that don't have decades of TTRPG experience. Eventually, folks come around and start wanting to know and understand the system. That, or they just want to kill a few hours with the friends and dont really care about the game all that much. You need to invest the time to tell which is which. No rule system, no matter how intuitive, is going to help the latter.
