It sounds like the sear is hanging up. There is a small spring behind it the pushes the bottom away from the hammer. The sear is #23 and the spring is #25 on the following schematic. When you pull the hammer back, the sear should lock the hammer back by locking on the trigger notch. With the sideplate off and the gun on its left side, pull the hammer back and watch the sear. If it does not catch, check the spring or the end of the sear for a broken tip.
You will need to call Taurus for any parts, Brownells does not have them for the Taurus guns. 305-624-1115 is the number for customer service and parts.