This is pretty vague. This problem is not unusual that combining fails meshing when individually works.
Here are some tips, but for sure I do not have time to explain in full details, because there are many.
-->It is usually related to geometry quality and mesh quality and settings.
There should be more feedback in the diagnosis when meshing than just this error.
Check tools - geometry checks to see if bad geometry
Check for geometry interferences and remove them.
Make sure model accuracy is absolute and consistent for all components.
Make mesh global tolerance setting absolute instead of relative.
Gradually assemble component by component to find the trouble component.
Make sure global simulation settings like default interface are not different between sessions that work and don't work.
If some geometry is imported and not native Creo those are first to check.
Check for short edges, sliver surfaces and fix them.
last resort - simplify geometry... start with removing most of the small rounds to make sharp corners. (unless in critical area)
Conventional meshing = loose approximation to supplied geometry = more forgiving mesh step and results more dependent on meshing.
Creo Simulate meshing = tight capture of supplied geometry = more difficult when geometry has bad conditions. but results less dependent on meshing.
