The DVR XP mechanism is only 6 parts. If it is on a stand we will probably need to take it off to gain access. Here's some pictures to let you know what it entails. There are documents attached on how to remove a headstock, but you don't have to take it all the way off. The instructions show how to get to the point where you can inspect for obstruction.
1) Is the lathe bed bolted firmly to the bench or stand? or is there an obstruction or debris between the headstock and the bed?
2) Is the headstock lock pin (below) correctly engaged in the indentation of the headstock swivel pin? The nut on the bottom of the headstock swivel pin is a shipping nut and is not required. If you find tightening it deters the symptom, the headstock lock pin may not be engaged correctly.
4) Does the detent lock lever (23 ) appear to engage in either direction or does it swing free back and forth? If lever engages, the pin (20) is probably bent or otherwise binding.
Is the detent pin seating correctly when the rotational position is selected? If not, it could be damaged or binding and causing the headstock not to lock in place.
If either symptom indicates it is binding, we need to remove the headstock and clear the jam, or repair/replace the pin and/or the spring (21).
If the detent lock lever (23 ) swings free, the mechanism is probably jammed/frozen. Remove the stand and lie the lathe on it's back. If you have or can find the shipping nut (or a substitute M8 course) from the bottom of the swivel pin (most folks never remove it), turn it out a few turns so a couple threads are exposed, place a hard piece of wood or plastic against the nut, and hit the strike sharply with heavy mallet or medium hammer, with the impact going into the swivel pin. The detent pin should snap free. Lube by hand with some T9.