Green Question Mark on OPC tags

I have two groups of OPC tags coming from two identical PLC’s. The first set of tags read fine. I copied and pasted the second set and changed their addresses to line them up with the second PLC but they all sit there with the green question marks and values of ‘null’.

If I browse in a different register to a new tag it connects OK, say N10:10, or N110:7… but it seems to not want to read any integer from N7, i.e. N7:1 or N7:100 will not read!!

Have done a gateway restart, has not cured it.
I am using 7.4.1

How do I solve this?

Found out the problem!!!
I had tags looking for registers that didn’t exist in the PLC
Was looking for N7:35 and the data table stopped at N7:25
Deleted tags above N7:25 and all others started to read correctly!