Loose knob on Marshall Major IV

Hey i just stumbled over your question and if it's not even fixed because you posted such a long time ago this is what i found.⏎
I just repair Headphones of a friend and he had beside of a dead wire what leads it to a non functionable left speaker, kinda the same issue, where the button was restricted in some directions. Could not skip back for example.
When it is the same problem, this will help you.
⏎
First you need to turn the pads of the right speaker side (or just that with the button) counterclockwise to remove them.⏎
Normally there should be a sticker around, when you press firmly around the sticker you should locate 4 screwholes.⏎
You could also peel it off when it's not a sticky mess or try to punch trough with any pointy object. I did cut them out with a scalpel.⏎

The cover i removed with "Jimmy" from iFixit or you can try another flat thing like a spudger or a thin knife.
It's kinda embedded in the case.

If you leave some kind of marks, don't worry to much, the cushionpad will cover them, just don't go crazy ; )))

Carefully pry from the bottom (the opposite side of where the cables leave the housing). It is sealed with some art of light glue and there are some pretty fine wires attached to the speakers and the circuit board. U really want to avoid damage them.
Kindly flip the cap over and look right into the corner where the button is, there is some kind of housing where the white ribbon goes in.
Might this bracket (blue circle) for the button got a lil loose, try to gently push it back in the guidance (yellow arrows (sorry, right arrow is slightly off, drawed it with my finger))

Reassemble everything nice and slow, keep an eye on the wires, so they don't
get somewhere between where they don't belong.
Like between the threads or some other sharp obstacle.

I hope this helps you or might some other one.
Maybe you could also put some superglue at the edges of the guidance or put some kind of Foam Tape that suit on top of the mounting bracket to help them stay in place.

Good success and best regards

José