Did this happen after a conversion from 7.9 to 8.0?
I also had similar problems with my UDTs when importing from 7.9 into 8.0. The fix was to delete the parameter reference in the UDT definition, apply the change, and then paste the same parameter reference back in.
See the existing topic, it may help with your problem:
http://forum.inductiveautomation.com/t/ignition-8-embedded-udt-parameters/24022/4