The most common reason why users assume there's something wrong with the pushback because it's not starting, it's due to an extra airplane constraint to the pushback, for example on many planes, GSX checks for things like wheel chocks, engine covers, embedded stairs, and these vary for each plane.
Of course, under normal situations, GSX will tell you about this and it's not starting because it's waiting for that condition to be solved but, if you either turned down all the GSX message in the "Verbosity" slider OR you turned off the Info message ( in P3D ), or if the top text green bar is unavailable for other reason, like another add-on using it at the same time GSX tried to send a message to it, you wouldn't see the message, so you wouldn't know what GSX is waiting for.
That's the most common case, since you haven't said anything about the airport used, or the airplane used, not even which simulator you use, it's difficult to say more.
