Perhaps this thread can help: I can no longer recieve sms messages
Your message buffer may be full.
You can interact with some functions of the modem with mmcli. Try:
sudo mmcli -m any --messaging-list-sms
This should list messages in your buffer that have not been fully received. Or so I understand it.
You can delete them with:
sudo mmcli -m any --messaging-delete-sms=[number]
Or you could try to read the data to a file with:
sudo mmcli -s [number] --create-file-with-data=/path/to/the/output/file
Deleting them should clear up the buffer and allow new messages to be received.