There's really no clutch adjustment for the power feed on the head. It's either engaged or it's not.
The jaw clutch that engages the power feed could be worn out to the point it will not stay engaged or the roll pin that holds the miter gear in place is sheared off. There are several woodruff keys in the gear train that could have sheared off, but I doubt it. You can still get most of the parts for this head from Index Machine company. Be prepared to spend $$$ for replacement parts.
I have the same mill that had the same power feed unit. It was gutted with missing parts when dad got the mill. He had notes where he contacted Well Index back may years ago for replacement parts. One of the castings and another part were untainable. After dad passed, I pulled power feed unit off and rigged up a hand feed unit that has worked fine for me for what I do. I still have a few of the parts laying around here somewhere. But some of the parts missing is the bronze worm gear and the clutch pieces. Ken