So much talk about the landing gear, but it is actually the door that doesn't open.
How much of the door is actually custom? After all, even on the test article, it should be designed to withstand re-entry heat. Something an F5 never need to do.
Part of that must be that it is completely closed. Superheated gases cannot enter, so it must be better sealed than a standard gear door.
Could this be the root cause? The door sealing?