This happened to me on the first theater level, and it was annoying.  There are some items that you have to find that aren't on the list.  These are items that point to one of the characters being a suspect, like business woman's day planner, etc.  Finding that item causes a character to turn up on the suspects list.
 
Look for something sparkly in the scene, something that could conceivably be an item belonging to a character.

I think they might also move more slowly if they are depressed or sick, but I'm not sure.