I cannot believe this has not been fixed by now!!!!
The issue seems to be that if the user saves an assembly and this assembly has parts or sub assemblies that need rebuilding and also saving, solidworks just bypasses the process to save a bitmap preview image.
The only way, I can see,is for the user to troll through the whole main assembly and open each part/assembly and save every one individually, so all parts/sub assemblies are up to date, then save the main assembly.
This is all fine, until a change is made on a part, say in a sub assembly, then that part is immediately out of date and again and those parts/subassemblies (all below the main assembly) loose their preview when main assembly is saved.
What a screwup!!!!!!!!!!!!!
This preview, surely, is a fundamental part of the usability of the program (or indeed any windows program).
When the user is sifting through many slightly similar items with no preview this become a PITA
This particular problem that Neville refers to has been reported as SPR#618533 and is documented in the SolidWorks Knowledge Base in the customer portal. This problem has been fixed for SolidWorks 2012 SP01, but unfortunately cannot be fixed in any earlier service packs (SolidWorks 2011 SP05 or SolidWorks 2012 SP0) because development on those versions is already complete and the versions are in final testing.
That is not to say that there could not be some other issue causing bitmaps to not display as there can be issues with setups on certain machines, etc. (as some other threads in the forum have discussed). However, if the problem relates to references files from an assembly or drawing not having their previews shown, it is likely this issue.