If the hand is moving the cylinder, but the hammer isnt moving back - the hammer sear is not engaging on the trigger. It's either gunked up terribily where its not resetting, broken, broken spring, or something of that nature.
That all assuming its not something more major like a broken frame pin that's messing things up.
You really need to open the side plate and inspect.
On the trigger -
Hand moves cylinder
Sear moves hammer.