I had the same problem with my comb. FWIW, it was not done by Greg Hissem. I made sure the comb was exactly where I needed it and put blue loctite on the set screws. Has been fine now for about 200 rounds. Don't use red loctite or you will never get the screws loose.
Use 2 shorter set screws, one on top of the other... or if you have the space put a short one on top of the existing one. The inner one should always be a bit longer. The inner one is tight, the outer just snug.
No different than using a jam nut to keep a standard nut tight on a bolt or stud. Only difference is the threads on the set screws are on the outside while the threads of the jam nut are on the inside.
Using the "jam" screw is the best idea I've heard of yet! Less of a PITA than having the comb rattle loose when beginning an important round and makes for easy removal anytime!! I lost two targets "thinking" about my loose comb in the first 5 targets at a recent shoot! Had to hold up my squad till I got my wrench out of my shooting bag and re-tightened them, then ran the rest. I'm headed for the screw shop so I don't screw myself again with this PITA problem!