I try not to give exact instructions on what to do when players are stuck, but sometimes they jump to more complicated conclusions and I'll hint that they might look at simpler answers first.
The only time I can actually remember giving them an answer is when I described an NPC drawing a rune to pass through a magical barrier and then quickly scuffing it out. They immediately concluded "He scuffed it out, and we didn't roll high enough perception to spot details of the rune while he was drawing it, therefore there's no possible way to get through this barrier and rescue our friend inside it." They were getting kind of upset and complaining that I'd put them in a "Make this skill check or you lose" situation, so I hinted to examine the remains of the scuffed out rune, which they were then able to recreate fairly easily. I think that was a situation where it was a good decision on my part to just give them the answer, rather than leave the adventure there with a sour taste.