Hi,
at DTP level how you set delta pointer?
If possible try to do at dev system.
1. delete data from target.
2. Trigger your delta dtp from psa. mode a delta.
3. Once the 2nd step was successful, once you got new request to psa, trigger your delta dtp.
its need to pull present request only. if it pulls latest request only mean its working fine. if not then you may need to dig further about delta issue.
if as said above if delta worked then same do at prod.if not then need check further more about delta pointer issue.
Thanks