Siemens S7/TIA v16: Monitored values don't match...why?

[Mas01]

In OB1, I'm monitoring some values (first pic). The values are what I expect.

At the same time, I'm monitoring the same values in the DB where they are written to, but instead of seeing the values, I see "????" (second pic)

Does anyone know why these do not match?

Thanks

 

[Mas01]

In OB1, I'm monitoring some values (first pic). The values are what I expect.

At the same time, I'm monitoring the same values in the DB where they are written to, but instead of seeing the values, I see "????" (second pic)

Does anyone know why these do not match?

Thanks

UPDATE:
In case it helps someone in the future, I clicked inside one of the pink cells and a message tells me that "the maximum number of monitoring operations permitted has been reached".

I tested this by adding another tag to the list to monitor it and it also appeared with "????" next to it.

I originally thought that the "???" meant the PLC did not know the value of the tag...this is not the case - the PLC has just reached a limit and will not display the monitored value.