I'm GMing some Shadow of Esteren games with friend. If you do not know that RPG just take a look it's very cool :-).
I'm following the scenarios given in the official book (don't have time to set up my own campaign and don't know the game much for now so it seems to be the best way to go) and sometimes the players are taking the "wrong path". Of course in a RPG there is no such think like wrong path and the GM must fall back on his feet to keep the adventure interesting for the players. But on the other way we don't want it to be too easy for them and not thinking about some crucial things mustn't lead them to the right answer directly, so I think.
The thing is I have difficulties to handle correctly such moments. I do not want to block players just because they don't take that way instead of another, but if they neglect some crucial element I don't want to simply make the way they choose the right way in order to give them access to the main plot.
I've an example of such a situation that happened before. They were following a poisoned river and were regularily testing the water after each possible source of poisonning) to determine the cause. But they forgot one of the possible cause, they didn't think about it. I let them going on their way but the thing is that at that point there were nothing really interesting and I tell them they follow for hours and hours finding nothing.
I feel a bit disappointed about how I typically handle this situation and I wondered if there was a better way?
Best Answer
I think 'the dark wanderer' and MC-Hambone have really good answers, but I wanted to add a few more suggestions. I read up on this specific scenario to get an idea of what the players may have missed, so I'll give my suggestions for how to handle it in this specific situation, and possibly how to use these suggestions more generally. Hopefully at least a little of this will be helpful.
What are the symptoms of the problem the players are trying to solve?
Think of the plot conflict as an illness that the players are trying to diagnose. If they keep thinking that it's Lupus (it's never Lupus), give them another symptom. First things first, when you read through the scenario prior to the session, keep in mind how the main conflict could be effecting the world. The official book should give you a lot of this, but try to think of any "unintended consequences" to serve as a fallback to give clues to the players.
In this situation: the plants along the riverbank could've displayed signs of being poisoned downriver of the poison source, but no signs of poisoning upriver. This way, if the players move past the source, you can just mention that they move into an area where the plants are lush and healthy. Don't specifically say "the vegetation here is not poisoned"...just say that it's beautiful glade, or has green grass, etc.
If they continue to move in the wrong direction, then mention that the most perceptive character notices that the plants there are not poisoned. This should give them a pretty strong clue that they have moved beyond the source of the poison and need to double back. If their samples keep showing up poisoned, then it should tell them that they have a flaw in their methodology.
More generally: regardless of how the conflict began, consider: were there witnesses? Could those witnesses have mentioned it to anyone? Would they seek out help? Would they seek out any of the player characters specifically?
If there were no witnesses, what sort of "footprint" would've been left in the immediate and surrounding areas? ie, did the responsible party have a campfire in the area? Did they leave in a hurry and forget to extinguish it? Maybe this caused a small forest fire that the players could spot, or even smell days after it was extinguished.
What's special about the player's characters?
Within the game world, why is it that these specific people are the ones who can solve this problem? Why can't the town guard, or random civilians, figure it out on their own? What skills/background/items/traits do the players' characters possess which make them uniquely suited to this task?
In this situation: If any of the players' characters hail from the region, they could have a childhood friend who might work in the place where the poisoning began, and would be willing to confide in the player's character. Maybe one of the players has a skill which might help them notice that the poison levels are too regular (if they aren't cleaning out the pot/vial they're using for the samples), such as magience, or a nature skill. This gives you an opportunity to simply say "You notice _____" if the players are going completely off-base.
More generally: At least glance through the players' character sheets beforehand (though ideally you may want to have a copy of each one handy during the game) so you have an idea of what skills/feats they have. If none of the characters has a high enough passive perception (or equivalent) to notice a clue, and the clue itself is vital for game completion, then consider how their unrelated skills might color their perceptions.
For instance, if the players are trying to track down some bandits, and they notice footprints heading north away from the site of an attack, they might just start heading north. If they don't think to actively track the trail (ie, they just say "we go north"), then they won't notice that the bandits turned east after a few miles. You can push them in the right direction by rolling 1d4 and stating that after that many hours, the character with the highest tracking/nature skill (or equivalent) notices that there is no sign of anyone traversing the area. This tells the players that they need to double back and try to find the bandits' trail, but also gives them a realistic little penalty for going off half-cocked.