When your Car A is taken offline intentionally or on a fault, and Car B tells you it's on "normal" but it won't take hall calls, check these;

1) Will the Car B take car calls?
2) On Car B, in the Spare Outputs Menu Options, look for these inputs to be high:
ISRT, ISV, ISVF (Each of these represents an "in service and ready" state).
If these are not active, your car won't respond to dispatching.
If this is the case, it could be software. It can occur on either car, rendering the supposed "active" car to be really not active at all, but everything will look absolutely normal from first glances. The suggestion to call MCE is a good one.