got a few UR1 transmission jobs done, but not a shuttle. As said, not many people ordered this option.
Have you drained the oil ? the shuttle shift is located in the front part of the gearbox, where in the standard version, the 540/1000rpm pto selector gears are located.
If you manually put it in gear and hit the starter button, will it reverse ? If it does, your problem is in the selector mechanism.
And if it doesnt, can you see e.g. if the synchro coupler cog wheel is stripped ? just try both forward and reverse, and see if the same shafts are rotating.
But anyways, try hitting the starter button with the transmission manually shifted in reverse to check first. a malfunctioning shift lever is way more likely and way less costly.