[LouAvul]

It's a timing problem. When you fire it "double action" (actually is "trigger cocking"), the momentum of the cylinder carries it forward to lock into place.

"Single action" (actually is "hammer cocking") the cylinder isn't moving fast enough for momentum to carry it forward.

Colt revolvers sometimes do this. The Anaconda model was especially bad.

Cock the hammer slowly. When it reaches full cock, then see if the cylinder rotates slightly until it locks into place.

Definitely don't shoot it and definitely find a competent gunsmith.

[Tijeras_Slim]

Bingo.