Downlink randomly fails

I have chirpstack all installed on a single linux device with a gateway and 5 devices (three dragino lt22222, 1 netvox R718N and one wise 4012. After testing for 3 weeka the gateway and end devices were installed in the field. It ran perfectly for 6 weeks then stopped sending messages to the end devices. Checked everything reset the devices and gateway but no join ack sent. Swapped the gateway with another of the same type and it started working. Dud gateway i thought. Failed again so bought a laird RG191 and installed. Worked for a week then failed with the same symptoms. Powering off the gateway for ten minutes clears the problem using the laird or dragino gateways. Nothing has changed on the chirpstack machine and uplinks get through no problems. At a bit of a lose as to what is happening. Searched the forum and found some close to my problem but not the same. Anyone with any ideas?

It looks like a network issue.
if your gateway connected to the ChirpStack over VPN, make start the packet forwarder after vpn is connected.
anyway look into gateway packet forwarder log to get more clear of your issue