Something similar happened to me, and I discovered that the gas cap retainage cord (cap keeper) had become twisted enough times by taking the cap off to fill the tank to actually put tension on the door so that it wouldn't stay closed. Sounds weird, I know. The plunger that actually pushes out the door and the door lock was working fine. This cap keeper was the culprit.

Hopefully that helps. If not, I'm out of other ideas.