Please provide with the actual download links for the affected liveries so I can check. Most common cause, assuming the livery is otherwise correct, is the name of the package it comes with not respecting the standard naming convention of the official liveries so, in case of the PMDG 777, to be recognized, their package must start with this pattern:
pmdg-aircraft-77w
So, it will work with both the base package, and the official liveries packages, which are all under this folder:
pmdg-aircraft-77w-liveries
If you have a 3rd party livery that is not recognized because of the package name, you might just have to rename it as such:
pmdg-aircraft-77w-my3rdpartylivery
Anything that start with "pmdg-aircraft-77w" will work.