This is a common problem with the J-12 on a FED 2. The answer is to bend to arm but be very careful and only bend it the least amount possible. Best to remove the back and offer the lens up to see how far to bend it. At your own risk, of course! I've had to do this on two FED 2s, it doesn't need much bending - I just used my fingers, not pliers.
Zorki: 1e (x2), 2C, 3M, 4, 4K, 5, 6
FED: NKVD, 1g, 2e, 3a, 3b, 4L (x2), 5B, 5C
Kiev 4, 4A
Yashica Minister III
Zenith C, Zenit C, Zenit E
Minolta XG M, Z1
"The way to see by faith is to shut the eye of reason."- Benjamin Franklin