Whenever a mystery needs to be solved for the plot to progress, you have the potential that the players reach a dead end. They might not have found the clues, they might have killed the NPCs rather than talking to them, or they haven’t put the clues together the right way.
It’s then up to the DM to lead the players back to the plot. However, it works best if you do it in a way that fits the story.
The roadblock my players had discovered was that they didn’t know where Cragmaw Castle was, a really important location to the plot. Between sessions, I scanned the adventure to discover clues that they hadn’t found. There was one, a druid, who could tell them where the castle was. The solution to this impasse was to direct them to the druid.
Could anyone in the adventure direct them to the druid? Yes, there was someone in Phandalin who could, but they hadn’t interacted with that character and it seemed unlikely that they would.
However, Sildar Hallwinter, the Lord’s Alliance contact in the town, and the one that wanted the PCs to find Cragmaw Castle, was able to interact with that character. It seemed possible that while the PCs were away doing some of the sidequest that he’d found that NPC, and learnt about where the druid could be found. So, when the players returned to town and visited Sildar, he conveyed the information to them. The players found the druid, and learnt where Cragmaw Castle was. We’re back on track!
I prefer, where possible, to use the encounters in the adventure to guide the players in the right direction, but it’s often not possible, especially when the adventure is designed so that there aren’t multiple ways of finding clues (a common design flaw). At that point, it behoves the DM to find new ways of getting the information to the players. Being at a roadblock and being uncertain how to proceed is terribly frustrating for players.
Do the PCs get help from a friendly NPC? Do they find a undestroyed note from the villain’s henchman that leads to the next important location? Does the cleric have a vision from their deity showing them something important? Yes, you could just tell the players, but finding a way to provide the information as part of the game is something I find preferable.
It helps if there’s a challenge for the players after they get this information. I was lucky that Phandelver provided that as part of the adventure; it wasn’t a case of just walking up the street to the druid’s house. Perhaps the NPC has been kidnapped. Perhaps the clue is in a small dungeon. Something to make the players feel they earned it.
As a player, you’re well advised to take notes and to review and order them after a session so you’re prepared for the next session. Having important information at your fingertips will help play the game. Of course, even taking notes is no guarantee of remembering them, but it does help.
It doesn’t matter how obvious the DM thinks the clue is, players will fail to find it. When this happens, work with the players to bring them back to the plot, and don’t let them sit frustrated too long. You can’t get to the epic finale if the players don’t know where it is!