Hi Steve,
Check in Content Manager that your assets are not open for edit and/or faulty.
For built-in and those marked as payware assets that are open for edit or marked faulty, highlight them, and choose revert to original.
For stuff off the DLS, any faulty assets need to be replaced or repaired, and sometimes a revert to original works for those.
For assets open for edit that are not built-in. You can try revert to original on those, but if that doesn't work try a submit. Right-clicking on the assets will bring up the option.
If all else fails, run a database repair, which is found under developer on the Content Manager menu bar, or even on the Launcher its self.