That’s a common problem that can sometimes drive you nuts!
There are several reasons this can happen. I’ll try to list a few here that I can remember and maybe others will also have some input.
1. Something called sympathetic vibration…kind of like the tail wagging the dog. If the case of the clock isn’t secure it may begin swaying back and forth with the pendulum. Even the slightest imperceptable amount can often stop some clocks. A lot of energy can be lost or transfered to the clock case when this occurs. That’s energy that is no longer transfered to the pendulum during impulse. It’s very important to eliminate as much of this movement as possible. This can happen with all clocks whether mantle, wall or floor. Mantle clocks are pretty easy to make stable in most cases just by adjusting the feet. Wall clocks should be secured to the wall both from the top (I guess that’s obvious) and if possible at the bottom. You probably have seen many clocks with adjustable pointed tabs that can press against the wall to keep them from swinging or wobbling. Floor clocks should be secured to the wall if at all possible. Especially if they are sitting on carpet! You can often step back from an unsecured floor clock and actually see it sway back and forth…not good at all.
2. Loose movement or movement feet. The movement must always be secure in the case. The movement tabs can sometimes become loose on the movement. Also you will sometimes find that someone has used shoulder screws in the past. This will often keep you from tightening the movement down fully. The repair is self explanatory.
3. Loose case back.
4. Warped case back and uneven movement feet… This can be a big problem as it may actually torque the movement when tightened down all the way! This torqueing (is that how you spell it?) may cause the moving parts to bind a bit. If you notice that the movement is running fine before tightening the case screws down but the pendulum motion or the amplitude (audio) of the tick lessens as the screws are tightened then your problem may lie in a warped case or uneven movement feet. If the movement isn’t perfectly vertical in the case then the pendulum leader may be hitting the back or front of the crutch which will also steal some of the impulse power. Also…is the pendulum just barely hitting the back of the case? Hard to tell sometimes without observation windows on the side of the case.
Well there’s a few…bet there is a ton more.