Always try a new, fresh battery. The clocks have two separate functions, time keeping and pendulum swinging. As the battery starts to fail, one function usually gives out before the other.
Check to make sure the positive end of the battery (nub end) is pointing down and the silk ribbon is not obstructing the connection between the battery and the mechanism. You may need to slightly pull on the silk ribbon to ensure the battery is making full contact with the mechanism.
Make sure the mechanism itself is vertical (straight up and down) -Taking into account some designs are asymmetrical and the mechanism may need to be slightly tilted to one side or the other to achieve proper balance.
Your nail or screw must be slightly tilted upward to hold the clock tight to the wall. (Nail should only be sticking out of the wall approximately 1/4".) Clock should be flush against the wall.
Make sure the swing arm is not hooked to the side post on back of mechanism. If so, gently push it to the left until it unhooks.