Transaction Group randomly can't see value

I have a transaction group on a project that has been running correctly for awhile. Randomly it seems that one of the group items can’t seem to find the tag value its linked to. It started to just log NULLs into the SQL table instead of the value of the tag it is linked to. I have a few different machines using this same UDT and they are working fine still. Is there something that I am missing?