If cleaning it doesn't fix the problem, remove the cylinder.
Cock the hammer, paying attention to the slot on the left side of the recoil shield.
The hand should extend out into space where the cylinder normally is.
If it does, use your finger to try to push it back into the slot. It should be spring loaded and push back until you remove your finger. Then, it should pop back out.
If the hand works as I described the problem is its upper surface is too tall.
If it is too tall, it may hang up on the ratchet face rather than falling into the next notch on the cylinder ratchet (on the rear of the cylinder).
A light filing to lower the top of the hand should fix the problem.
If the hand doesn't extend or if it doesn't pop out when it is pushed into the slot after removing your finger, either the small spring on the hand is broken or the pin that connects the hand to the hammer is broken.
In either of these cases, buying a new hand from a place like Dixie Gunworks and installing it should fix the problem.
New hands often are not a drop in part because although it will fit into the hammer and it will extend like it should, often the top surface is too tall to properly engage the cylinders ratchet.
As mentioned, this may keep it from engaging the ratchet or more commonly it will try to turn the cylinder after the cylinder bolt has engaged the notches in the cylinder.
Filing the tip so it is shorter will usually fix this.
