DIAGNOSIS The self-cleaning latch mechanism is likely binding. There's a mechanical latch assembly on the door that engages for the clean cycle, and its springs or levers can get gummed up or wear out.
FIX Inspect the top and bottom of the door frame where it meets the range. You'll see the latch hooks. Clean any visible debris from them and the corresponding slots on the range. Apply a small amount of dry lubricant (like graphite powder) to the moving parts of the latch mechanism you can see. Work the door open and closed a bunch of times.
IF THAT DIDN'T WORK If it's still sticking, the entire door latch assembly (part # DC97-22329A) might need replacement. It's a known wear item on these models.
HEADS UP Don't use WD-40 or oily lubricants. They'll attract more grease and grime, making the problem worse in the long run.