Car 1 is the one one the 4th floor and Car 2 is at the Lobby. Car 2 will answer calls and go anywhere. You can send it away and Car 1 won't come down. If you go up to 4 and register a hall call and get in car 1 and register a car call below 4 it won't go down(so they say). They have to register a car call above, the car will go up then come down. Only thing found so far is that on Car 1, DCL wasn't opening sometimes when the door was fully closed so GH would never drop and DG never dropped either. So far it's been working for 2 days in a row. FWIW the cars don't have the original door operators.
You would have to establish that xd is not coming in and if not it sounds to me e-man that you may have a fh hook contact open. Can you get them to test going up to each floor and test ie by checking the floors on the way up you can isolate which fh contact is actually open. It could also be the bottom selector cam flex that is open cct at that point. Easiest way to test this to clip a short to it and then test it.
