Cursor Won't (pre-)select edges, faces, etc.

I have found that very occasionally, something happens and my cursor will suddenly not pre-select an edge... what I mean is that as I move the cursor over a part, none of the edges or faces highlight.  Understandably, this makes it far more difficult to work in assemblies mating parts and such.
Any idea as to the fix for this?

Thanks!

SolidworksDrawings And Detailing