I've had one of these for several years now and have never once had an issue. Now all of a sudden I cannot turn the nock receiver AT ALL. I've loosened all three screws and nothing works. I'm stumped. This has never happened before and is pretty frustrating.
Any ideas?

I'm willing to bet that some how you got some glue in there. I've never had a problem with mine either, unless something just got loose and in a bind.

Being as you've had it for a while you obviously know how to adjust things. There is a set screw that indexes the nock receiver. Loosen this or may even remove it. Be careful if removing it as there is a little spring and ball bearing under it that cam pop out and get lost. Once you have it loose or out you should be able to turn the nock receiver. If you can't then grab it with a pair pliers and try it. Twist it back and forth while trying to pull it out of the jig. Then clean up the surfaces with steel wool or scotchbrite. Rub a little grease or even bowstring wax on the receiver and reinstall it. When replacing the indexing screw don't make it too tight.

I too suspect its glue. As mentioned be careful of the index/detent springs and balls. Hold the fletcher upside down, should keep them from falling out as long as all spring tension has been removed. Don't forget, besides the index/detents there is a slot head retaining screw that keeps the receiver from just sliding out of the fletcher. It will need to be removed.
I prefer using silicone based grease as it doesn't "creep" and form a film all over everything as time passes like petroleum greases do. Onc eyou get it apart and all cleaned up, I think using the string wax is probably a good idea as it will most likely be silicone based.

Thanks for the tips everyone. After popping out the nock receiver and applying some lubricant to it (lots of it actually), I got it turning again. It took A LOT of twisting and working the receiver back and forth to get it moving, but it finally got to a point where it could turn freely again. The stops at the three fletch positions seem quite a bit looser than they used to be, but at least it's working now.

If the "stops" are too loose you just need to add a bit of tension to the the detent set screw. Sometimes just a little. Glad you got it figured out.