The trigger is not going far enough forward to reset. The hammer sear needs to be on top of the trigger at the back and the cylinder stop needs to be under the trigger at the front.

Push on the back part of the trigger where it's in the trigger guard and see if it will go forward some more. The trigger rebound spring may be too light and not pushing it all the way forward. Sometimes the spring is cut off to make it lighter. If that is the case, put in a new rebound spring to fix it.