GX450 not reconnecting-Sprint


I am testing 3 Sprint SIM cards in GX450 modems. We normally use Verizon. With the 3 modems testing Sprint, there is an issue when the Sprint coverage drops and the modem loses connection. The modem often does not reconnect to Sprint unless the modem is rebooted. This is in areas that I know there is excellent Sprint coverage. I loaded latest Radio Firmware and ALEOS software from Sierra for Sprint. Any ideas?
Thanks so much!