Partially loaded/grey avatars over a long period of time are usually caused by packet loss/connection issues or LL-side communication/asset issues.
A solid workaround is to change your group tag. For some reason, this forces all avatars nearby to re-rez.
For those that don't rez, try going out of viewable range (fly up real high maybe) and then come back down.
Failing all of that, the odds are that the texture baking is borked on their end, and a polite IM to ask them to rebake usually works great.
Using a combination of these three steps, I never see grey avatars. Good luck.