There're a number of ways to resolve these issues.
Sometimes just a simple quit VG and then restart does the trick.
Then there's Right click on the Synchronise with server in Advanced tab and either Rebuild All or Register Again (You need your password and login).
If you're seeing your cards all greyed out (as tho not yet downloaded), then select them all and right click the trashcan, then check for updates.
Occasionally, re-installing the software might fix these issues too as sometimes you may have corrupted files. Just overwrite your existing install. Do NOT un-install the software or delete your models or data folder. If you have back-ups of all your cards (models and data), you can delete the current ones and then copy the back-ups over. A simple re-start of VG should then show them all :)
If all else fails, then it's probably time to contact Customer Services with full details of your problem, including your set-up details, OS etc.
However, I will say that with any problem I've encountered over the last 5 years, with the software or any other issues relating to VG, I've personally NEVER had to contact CS to fix an issue. Most things have already been addressed somewhere here in the forum by somebody (usually in Bugs and Fixes) and a little digging will usually sort you out :)