Incremental seems to be doing full backup

Currently using backup 4.4.1
I have a backup scheme setup with a full monthly backup, weekly differential, and daily incremental. These are all saved as the same cbu file on a NAS so that I only maintain a single file, and then this is renamed and moved before the new month’s full backup starts. Until recently this seemed to work well, but lately I’ve had some issues. There is almost always a dialog box that says “Error: could not run job” (or something similar, I don’t have it in front of me now but I will update when I next see it). Also, the incremental backups which should take very little time as there is never more than 2GB of new files on any given day goes for hours. Currently I’m 54% done and it says I have another 19+ hours remaining! Everything says processing new file. Screenshot of current backup and of list showing a successful backup and size.
Any ideas?
Thanks in advance

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