Backup failure on Netbackup clients

I'd try the suggestion of copying the policy to a new name, e.g. copy POLICYA to POLICYA2, and deactivate POLICYA.  I suspect that the policy execution manager may have become confused and is no longer able to run certain clients in the problem policy name.

I'm curious as to whether you are able to run the "skipped" clients manually.  If you are, then you need to go back to the manuals and ensure that you fully understand how this affects the "schedule window", and that by running a backup manually it is entirely possible and logical that such clients will not run again at the start of the next "schedule window".  i.e. it is quite likely that client(s) will be skipped BECAUSE a manual backup has been run.  Another term that you can research in this forum is "schedule creep" which refers to mis-configured schedules that appear to delay starting backups, or miss/skip them entirely.

If you really want to determine the bottom level root cause then you'll need to raise a case with Symantec Support, as this forum isn't really the place for detailed technical analysis - it's more for quick workarounds to get you going again.

BTW this forum is not an official Symantec support channel.  To post here is to have no expectations of any response whatsoever.  It may be "kindly" hosted by Symantec, but that's it.  It may be frequented, from time to time, by Symantec employees "helping out", which is very kind of them, but don't "expect" a response.