I have been using Solidworks for about a year now, and am currently using version 2015.  I've been having this issue for quite a while now, but thought I'd ask if there was some reasonable explanation for it.  I'll work on my files and assemblies and they seem to work fine - all mates are valid and things are good.  Then one day when I open them the mating conditions are greyed out and they no longer appear to be functional.  The parts float in space and can be moved around as if there are no mates applied.  In order to get them to work I have to open each one up and re-select the faces or edges.  Image of the greyed out mating conditions is attached.

Since this takes quite a bit of time and is very frustrating, I was hoping someone could shed some light on this issue.  Any help would be appreciated!