If the Matrikon OPC Connection is broken between the PLScada Server and SPM Server, check the following
To verify if the connection is broken, go to Matrikon Explorer and click on the Tunneller connection. For example if you have PLScada running as the Primary OPC Server then you need to go to Tunneller:\\ServerName\Citect.OPCRemote.1 then click on connect then check if the connection status says running or failed. Do this on the primary side where you have the Primary OPC running. If it is saying running then go to the other server and go to explorer and try to read a tag.

If the connection on the Primary OPC server status is Failed or cant browse to the tags on the second server then:

1- Make sure that the PLScada project is running and the SPM services are running.
2- Make sure that retries are setup on Matrikon so that if the PLScada project has been restarted or one of the IOServer is not responding then Matrikon OPC will retry to connect after PLScada is running normally. Also make the timeout set to 5 minutes or more to give to PLScada enough time to restart.

These are the steps to change the retries and the timeout settings:

a- Go to start menu --all programs---Matrikon OPC--Tunneller---Server Side Gateway Configuration
b- Go the Advanced Tab and change the settings in accordance with steps above.