So, with subscriptions loggers at DEBUG level you should be able to follow the entire flow relating to each Item, starting from the subscription request to the adapter and all the updates received by the server.

Message like this refer to the subscription request forwarded to the Data Adapter: "Subscribing to item <Item> on <AdapterSet.DataAdapter>"

Then you should looking for the replies of the adapter with messages like these:
"INCOMING SNAPSHOT for <Item> on <AdapterSet.DataAdapter> --> ... "
"INCOMING END OF SNAPSHOT NOTIFY for <Item> on <AdapterSet.DataAdapter>"
"INCOMING DATA for <Item> on <AdapterSet.DataAdapter> ... "

With that information you can be sure whether the updates are flowing regularly from the adapter to the server. If so, we'll need to focus on the client side.

Regards,
Giuseppe