You might find that the engine data and transmission data are two different node addresses on J1939. Mine were with my Deere engine. I think the Maretron config software allows you to scan the J1939 side via the J2K100 config page to see what devices are present. If you find another device, try setting up the J2K100 to use that device and see if trans data now passes through. If so, it will confirm that the data is there, just under a different mode addess.
If you find the data is there, the unfortunate thing is that you will need a second J2K100 to then display it since each will only pass data for a single device. You set one up for engine data, and the other for transmission data. Annoying, to say the least.