OPC UA Client .NET - subscription stops after a certain time or event|OPC UA Implementation: Stacks, Tools, and Samples|Forum|OPC Foundation

So in OPC UA a server uses the lifetime count to determine how many times it should cycle through a the publishing intervals with out being able to send a publish response when it has data to publish. The way publishing should work is that the client should send 1 or more publish requests to the server for each subscription.On a publish interval if there is data for a subscription the server will dequeue a request and send a response, if there is none then on the lifetime count it will send dequeue a request and send a status update letting the client know that the subscription is still alive. If the client has no publish requests to dequeue it will eventually cancel the subscription at which point the keep alive reuqest to get the server status will succeed but there will never be another publish response. You should be able to do a registered or unregistered read of the sever and get data though. The client should always send a new request when it does get a response to make sure there are always enough requests queued.

Possible causes can be that the client does not send enough publish requests initially or in response to responses and the queue becomes empty.

The client does not send subscription status requests which will tell it if subscriptions are still active in the server. (Most UA clients do not do this.)

The client did not renew the session before the session timeout.

I would get a wire shark capture and look at what is happening. You should be able to see what I am talking about. Wireshark will decode the UA Packets if you are not using encryption and you know the port number of the endpoint.