The problem has been identified and 99% solved. The problem was that the hop up rubber/bucking was enlarged or changed in a way that the hole got tighter so the BBs couldn't pass through; gas couldn't past through it, but you can still push it through with a stick or something, which took some force to push through.
After some fiddling and forceful installation, the hop up tore. I put back what's left of the hop up back and reassembled the gun to see if it still works. It does. Semi & full-auto both work. I have ordered a replacement hop up, but this will do for the time being.
Thank you all for your input.
Here's a piece of the hop up that was torn off (by accident):
