To troubleshoot those stales tags, my first step would be to check their scan rate. It could be that the tags have simply a slow update value (e.g. once a day, or once a week) and therefore are showing stale when in reality they are normal.
Next I would check those values in the data source, to confirm that there is nothing wrong with the instrument.
If the tag looks good in the data source, I would than troubleshoot the tag configuration, to check if I don't have a bad instrument tag, variable type, compression and exception parameters.