The TRS length varies in the female receptacles for male plugs 1/4-inch TRS (stereo) cables. I've seen the problem with Behringer and Donner headphone amps and Hosa 1/4-inch TS female to TRS male adapters.
You get only one channel unless you back the male plug out partially.
How do you get TRS male plugs that fit the receptacle yet match the TRS bands?
I've been observing that variations in tolerance is a bigger and bigger problem, with many plugs being made apparently by cheap manufacturers, even for equipments of supposedly good brands.
Just the other day I had to sandpaper the tip of jack as it refused to enter into the plug (it was ok after this treatment :-).
Regarding the actual question, apply any type of material that you find suitable around the sleeve part of the connector, near the base, so that it makes a cushion with the required size to stop the jack in the proper position. Wire, cotton string, duct date may all be a make do solution, however a wire will probably be best as you can adjust the thickness of the cushion by the number of turns and it will not deform (so much) with use.
Another alternative is to find a steel washer size that fits into the jack and does not protrudes from the handling part. With a bit of luck one or two washers (depending on thickness) glued in position will do the trick.
If you're a handy man you can find a bit of material (e.g. plastic, or even wood) and make your own washer with the exact size specification to fit the jack and make it stop in the precise required position.
Cut a washer out of something non-conductive (actually, wouldn't matter if it WAS conductive I guess) and slip it over the shaft of the plug.
