I think this a common problem
I believe this subject has appeared here over the years and therefore not uncommon. As I write I am looking at the head door open and see 2-3 inches at the top and and 4-5 at the bottom. In this case the top"header board" on the head side of the bulkhead (this is not redundant, really)on the starboard side dropped about an inch from the overhead. It has been this way since we bought the boat (7 years), with no noticable change. The main headache is the door jumps the track. We have hung a curtain for privacy. Andy"Baroque"