Thanks all for the info.
We are using MII version 12.2 and don't have any immediate plans to upgrade to 14.0.
The function module works fine in ECC se37 when executing with the same data and I have verified with the abap programmer that all required fields were sent.
I also cleared the jCo cache using http://<IP>:<PORT>/XMII/JCOProxy?Mode=Reset
We have decided to re-design the custom RFC to not use nested structures.
Thanks