Is it an investigation adventure? I've never really seen a complex adventure that doesn't force the PCs along the path. That's why I'm curious about the style of this series.
L2 (The Assassin's Knot) doesn't. It's an open question regarding how effective it is at doing mystery, although AFAIK it was the first published D&D mystery module, which (a) means you should cut it some slack, and (b), means you should cut no slack to newer modules that don't do it as well, particularly given new tools like the Three Clue Rule. (Now that I think of it, L2 started by handing the players three clues to get the ball rolling.)
I note that L2 tells you exactly what happens if the players fail to solve the mystery in time.
I think that a lot of the early games assume PCs are just going to wander around figuring things out. Like, in 1e the domain game supports clearing out the monsters and building your stronghold, but not a lot else. But we know people played in those games, so they must have just done a lot of interaction with NPCs and made domain-level decisions, which was all adjudicated by DM fiat because there were no mechanics. Other than the "Assassin Spying Table", the existence of which which probably says something about how players and NPCs often approached these situations.
I do think the DM needs to be generous in handing out clues, or letting the players know the significance of what they do find, particularly if the significance is setting-dependent. For example, if you and your players are living in a landlocked jurisdiction like Alberta, and you are running a find-the-kidnappers mystery in a port city, you may need to point out that the wharf is where the kidnappers are likely to go if they want to smuggle a captive out of the city..
And the players just don't know the setting as well as you do, or as well as their characters do, and are going to need a little (a lot of) help. If I say that the kidnappers have Karrnathi accents, I'm probably also going to have to hand out the fact that Karrnath is a nation on the northern part of the continent, tell them a few things about why Karrnathis might be interested in kidnapping the person, and advise them that the only realistic way to get to Karrnath from here is by ship. Preferably indirectly, but it has to be communicated in a way that they recognize it's significance. Hopefully that is enough to get them poking around the docks, but players seem to have a very hard time with analysis paralysis if clues are not super easy to identify, and also very easy to draw a conclusion from.
I'm thinking about it now, and realistically, the only way is to give them some contacts who can help them interpret the evidence and give them suggestions about where to go next. Prefereably three of them.
[Also, what is up with the spellcheck in this forum? "NPCs" is a problem but "suggestiongs" is just fine?]
