The TX led on your unipager/pi_hat is not blinking but working all the time? Maybe too many messages are waiting to be delivered and the queue is filling up.
Learn here how to check the queue (for unipager and pi-star/DAPNETGateway) and how to empty it.
Goto http://unipager_ip:8073/status and you will also see the number of queued messages:
„sudo service unipager restart“
All queued messages will be lost.
As root: „service dapnetgateway restart“
All queued messages will be lost.