Do you have one session open spread across both monitors? Do you have it set up so you have 2 viewers open with one on each monitor? Do they disappear if you have different sessions open on the two monitors and you revert back to showing on only one monitor? Do you have it open on both monitors and when you open a new part or assembly it just comes in blank? Do you have an assembly open across 2 monitors and some of the parts are not viable? Does this happen in a predictable pattern or is it completely random?
So many questions.............so few clues.