I am having trouble reading some tags from an Allen Bradley Compact Logix PLC into ignition.
Ignition Designer indicates that the tag value is null and the quality is unknown.

Going onto the ignition server into OPC Quick Client I can manually read from a tag, which gives me its correct value (checked in the PLC code), but when I click the subscribe button it tells me the quality is stale.

Why would reading the tag be fine but subscribing results in a stale tag?

This issue only occurred recently even though Ignition had been communicating successfully with the PLC for months

I am using Ignition 7.9.5