Players not sharing vital information with the group is one of my biggest frustrations as a DM. Especially when you are running an adventure in a system that relies heavily on clues, such as Call of Cthulhu, having a player hide that all important clue can seriously derail the adventure, or worse: get the whole party killed.
There are only so many clues you can drop as a DM, and if one of the players is just carrying around the clue of where to go next, without looking at it or showing it to the rest of the party... argh, it makes me want to pull my hair out.
And I can drop in reminders of course: "Remember, you still have that black book in your inventory that you haven't looked at", but when even those reminders are ignored... then what do you do?
I don't like railroading my players, but I also don't feel it is fair for the whole party to fail the adventure because one player was holding back an important clue.