Intermittent AT SMS operation

Using a Airlink GX400 and standard “AT*SMSM2M” command on a serial port. Always get “OK” reponse, however, messages are totally lost for variable periods of 20 minutes or more when the system is first rebooted. After this period only new messages are correctly received, older messages never arrive, even allowing for the possible 3 day delay of the GSM SMS system. AIrlink unit always show a good signal strength and connection.
Any advice will be greatly appreciated,