FBA listings not getting Prime badge

I noticed my FBA sales had fallen a bit and there had been a bit of a bump (though not as much) to my FBM sales.

When I checked my FBA listings I noticed that the Prime badge was not being displayed and that delivery was 3 days (longer than FBM which is why the FBM sales had picked up).

I contacted SS to ask what was going on and they gave me some line about all FBA offers getting reviewed from time-to-time and that while this happens Prime goes away.

I can't say I was convinced about the logic of this. Has anyone else come across this?