Transactions not getting posted in Vertex but OK in SAP

Hi all,

Am into an issue as follows:

I have 2 IRs posted to a PO. The tax code on the PO is AP Tax from Vertex but was changed to AP self assess tax in both invoice. One of the invoices got updated to the external system (Vertex) but the other one didn't. Compared the IRs side by side and they look exactly the same and was posted by same user.

Does anyone know about any process which happens for a self assess tax posting. Also the behind details of how a posting from SAP gets updated in Vertex.

I would really appreciate any answers and would assign points for the helpful.

Thanks alot