Since you are not providing the most important information that we always ask when reporting such issue, which are:
- The airplane used
- The scenery used
- The gate used
We can only guess. Usually, when pushback doesn't start, it's because you have some constraint specific to the plane, like wheel chocks or similar. GSX will obviously tell you about that but, if you turned the Verbosity slider in the GSX - Setting too low OR if you increased the time between message to a very high value, you'll miss what GSX is trying to tell you.