Sounds like something is definitely broken. It's a pretty straightforward design. As you rotate the knob the rotating screw threads inside move the nut inward or outward and bring the muffler closer to the head or move it away. I'd say either the rivet that acts as the fulcrum of the lever action is broken or else the nut that rides on the threads is stripped out. Possibly the threaded rod has detached from the knob but then the knob would probably fall off the drum. A photo or two would help with the diagnosis.

I can post some photos, but nothing looks abnormal. The rivet is intact, and the screw still threads fine.

Very odd indeed! As K.O. mentioned, a picture will really help us out.

-Mark

The only other thing I can think of, if both of the above mentioned parts are intact and working, is either the whole assembly is installed upside down or else the two little screws that hold the base plate to the shell are missing so when you turn the knob the whole assembly rotates. In either case you'd be looking at the two small screws adjacent to the knob (on the outside of the drum they are hidden under the knob itself).

Get us a picture and we'll figure it out.

Hopefully these help

If it literally falls, like spring-bouncing-around, knob-pulled-out-of-the-drum, that little brass bit is stripped. And there does seem like I see a fair amount of metal in the threads between it and the shell. You'd need new guts for the muffler.

If it's a controlled, you-tighten-it-and-it-moves-toward-the-reso movement, I think when it was last installed it was past the tipping point in rotation where it swings down as it's pulled closer to the shell. You'd need to loosen the knob all the way, tip it upward towards the top head, then reassemble so it's swinging that way.

ETA: Also looks like the muffle pad surface has gotten bent pretty good. It should be much closer to in-line with the arm than the 90-ish degree angle it seems to be.

My new best guess...the muffler is installed on the shell upside down. Pull out the two screws holding the plate to the shell and flip it over and give it a try.

Okay, after looking at your picture on something bigger than my phone, and taking a look at one of my own RB drums it becomes apparent that someone has bent the arm that holds the felts on your drum. It isn't upside down, the pads are just bent 90 degrees. probably by someone who didn't want to use it and having it in the "off" position wasn't good enough.

Could you post a photo of the proper angle because this is the first time the muffler has been a problem.