My advice on that front is just to have more clues than you need, and so if they miss a few juicy ones it's not the end of the world. You can tell them after they solve the Case! There's a Reconstructure expertise, if I remember my own writing correctly, that also lets them know if they haven't found any clues you had pre-written.
If they absolutely need a clue, force it (offer them a roll where no matter what, they find the clue, but a bad roll stings a bit) as discussed above. Missed clues are actually good, because they mean the players genuinely get to carve out their own path through the mystery - as long as there's enough clues to get there in the end. You may have to generate some on the fly to make sure, but again that helps the players take ownership of their solution.