So I'm running an official 5e campaign adventure, which shall remain unnamed so we don't get into spoiler territory. I'm primarily looking for general advice and the points of view of other GMs and players more than specific fixes to the adventure.
Here are a few recent situations:
1) Party needs to get into room to defeat evil caster who is terrorizing the local village. Caster is safely locked behind a door that (per the adventure) "cannot be damaged, forced open, or opened in any other way besides a knock spell (or a second spell that is fairly obscure)." All the party can do is leave the quest incomplete.
2) Party enters the first room of the dungeon. There's a monster that is resistant to magic and immune to non-magical weapon attacks (and has a boat load of HP). Party doesn't have magic weapons, because none are placed in the adventure. All the party can do is leave the quest incomplete.
3) Party fights their way through a dungeon to get to the BBEG. He cannot be reduced below 1 HP unless the party casts one of two spells in another room that they are too level to be able to cast. (The adventure specifically says that no other actions work.) All the party can do is leave the quest incomplete.
There are more examples in this adventure, and it's certainly not the way I'd design my own games.
Would you tinker with the adventure to give them a fighting chance? Also, when following the milestone XP suggestions, they get nothing for incomplete missions. So they can't level up to be able to complete the other missions, stuck forever at 4th level.