WIZ550S2E TCP not working on corporate network. loses first 16 bytes of data.

Hi, I am new to the forum, but have been working with the WIZ550S2E with version 1.4.0 firmware for a few months.
I am using the module in 2 application in different modes, both use DHCP.
Mode #1 UDP in Data Mode
Mode #2 TCP in Command mode, using AT+NSEND

On small networks both modes work fine. On a corporate (domain) network with a Windows DHCP server Mode #1 works fine but in Mode #2 I am losing the 1st 16 bytes of the data packet. This Wireshark trace shows the problem:

This trace shows what should be in the data block:

Does anyone have any suggestions as to what is causing this data loss?

Thanks

John