Different bellows technique?
I have experienced tight bellows on both of my new accordions. I loosened them up by opening the bellows about 1/2 way and then flexing the bellows in all four directions, without releasing any air, to their extreme movement. You should hear a slight "crackling" noise as the bellows stretch and flex off the tightness (this only happens on the initial flexing). Before starting a practice session, I also would open the bellows as far as I could manage and then hold them in the open position (they will want to naturally close to a neutral position, HOLD them open) for as long as I was willing (10-20 minutes). I did this primarily to reduce the opening resistance of the new bellow, but it improved the resistance factor dramatically.
Also, try opening the bellows only at the top, pivoting off a completely closed bottom. Then when bellows reach their maximum "opening", roll the bellows into the opposite orientation (fully opened at the bottom and closed at the top). No compression, just a "rolling" movement. Now the bellows will be in a position to take advantage of the pull of gravity. Add necessary compression with the left arm to achieve the desired volume and repeat once bellows reach a closed position. May take some practice to get the "roll" natural and even, but gravity is a force to be reckoned with. I used to have an excellent example of this bellows technique, but couldn't find the video link. Dingo?
You might start this technique by leaving the bottom bellows strap attached and just opening and closing the top half of the bellows in order to get the feel of the first half of the movement, and then progress to the "rolling/rocking" movement.
You may benefit from the hemp extract CBD (not an accordion type) rubbed on the effected area a few minutes before playing. The stuff is amazing for pain relief (and won't get you high,

).
Press on.......
Waldo