I had exactly the same prb. and resolve it as follow:
GSX options - Simulation - Display only important message.
(It was on Display all messages)
Then you must have seen some kind of message saying why you cannot start the pushback. In fact, if you do exactly the opposite, and disable some messages, you might miss something GSX is trying to tell you about WHY it cannot proceed with puhsback.
The PMDG 737, for example, has the following constraints before pushback can start:
- WheelChocks must be removed
- The PMDG GPU and Air Start Unit must be removed
- The APU must be turned On
- The Pitot Covers must be removed