I am sure we all have a different idea what is the right amount of "downness" on the stabilizers. Barely touching the ground? Tight to the ground? Maybe lift up one side just a little to get it really level?
As we were learning just how far to put these things down, it seemed sometimes the door deadbolt was hard to lock, other times not. Perhaps we had put some jacks down too much and twisted the trailer just enough to misalign the deadbolt, but not make the door hard to open or close. Perhaps when one person does one side and another the other side, each with a different idea of how tight to make them, that can twist the door frame just enough for a problem. I don't know how much flex there is to the frame and how it affects the door, but perhaps a bad road bump one day can knock the frame silly, or knock it out of line for a day or two. Sometimes it has seemed easier to close the door than others—temperature changes, twisted frame, or just imagination?
I also found the deadbolt mechanism needed some grease inside. That helped too.
We were advised when we picked up the trailer to not slam the door and to push it closed firmly, but carefully, at the spot where the handle is because it is reinforced there.