My ceiling is very flat. I painted the ceiling semi-gloss white. When I roll back to remove the lines that are left the roller leaves flat spots. P.S. My roller cover is not a cheap cover. Please Help
Sounds like you are dry rolling, at least a little. Maybe the dry roller cover is removing paint as you go back over the painted area. Try re-loading the roller before going back and work out the paint.
Either it is dry rolling or a worn out roller frame and the cover is sliding on the paint.