On my D2 if I don't slam the back door and it doesn't fully close then neither the external nor internal door handle will open the door. The quick fix is to give the door a good firm bump with my shoulder or a decent thump with my boot sorts it out.

Clearly I need to adjust the rear catch but present solution works! Just looking at the door it is not obvious if it has not fully closed and engaged the catch properly but when I push hard on it you hear it click into the correct position. Don't know if the D1 could have same problem.