I assume you mean you have a ULT installed. Or, do you mean you are just using ULE rail and aluminum body? There are a few things it could be.
If the trigger rod is in contact with the back of the trigger when you hold the trigger against the safety, it means the trigger rod is too long and won't allow the on-off to open. If there is a really large gap then it is too short, and the sear won't rotate far enough to release the bolt.
If the level 10 bolt isn't tuned so that it uses the largest carrier that doesn't produce a leak, it won't fire when you press the trigger and rotate the sear to release the bolt.
Check the velocity setting. A velocity setting that is too low may not have enough pressure to push the bolt forward.
Make sure you have enough shims installed in the ULT. Too few shim and and the on-off might not open when you release the trigger.
Except for the Automag in front, its usually the man behind the equipment that counts.