The disk backup job works, but doesn't seem to clean up and start again on Monday.
Yep, this isn't not how the retention policy works, as it keeps specified number of restore points, rather than deletes existing backup chain and starts it anew.
however when Monday comes around, it doesn't do a full backup to tape only an incremental and uses a synthetic full on Monday.
Could elaborate on it a little bit? You mean corresponding tape job doesn't copy full backup? Or it copies only newly created full backup not the chain as a whole?