Warp Stabilizer not Working

If you want to change the frame rate change the interpretation of the footage. If your original footage was 120 fps and you want it to playback so that one second of real time is 4 seconds of screen time then interpret the footage as 30fps. If you want to work with 24fps comps then interpret the footage as 24 fps. Then you can create a new comp from your footage and run warp stabilizer. As Byron said, the footage layer you warp stabilize must match the frame rate of the comp. Stretching footage is an inaccurate way to create slow motion. Your first step should always be to change the frame rate using File>Interpret Footage>Main. This will always preserve every frame. Stretching footage will cause frame blending or frame doubling.

If on the other hand, your original footage was shot at 24 and you want it to slow down you would get better results by using time remapping and some math so you get 2 frames for 1 or 3 frames for 1, or at best 4 frames for each frame. Then you can use AE's various frame blending modes to try and achieve the best motion. Slowing down most footage more than 2 frames for 1 so that one second of real time is 2 seconds if screen time is a little dodgy. Any slower than that and it starts to look bad. Any slower than one second of real time for 4 seconds of screen time is awfully hard to pull off without a third-party plug-in like Twixtor.

There is one other thing to consider when using warp stabilizer. Your footage should be trimmed to just the frames that are going to be used in the final edit. You can add a few frames at the head and tail so you'll have a little room to fine tune the final cut, but warp stabilizing an entire shot is almost always, a waste of time, more likely to fail, completely unnecessary. If you are going to do any other processing on the frame that is complicated or has more than one or two applied effects it is almost always a very good idea to render your warp stabilized footage to a suitable production format. Once rendered you can get rid of your warp stabilized layer and replace it with the repaired footage.

The last point I'm going to make is that not all shots will warp stabilize and most that will an be improved if you learn how to fiddle with the knobs. Warp Stabilizer's default settings only work well for nearly perfect for stabilizing shots. Most of the footage I've seen that need warp stabilizing need some fine tuning.

Oops, one more point. If you need to run Camera Tracking, motion tracking or use Mocha AE to do some tracking this always works best if you do the tracking first, finish the composite, then nest the completed comp in a new comp and Warp Stabilize the nested comp. Warp stabilized footage almost always fouls up the Camera Tracker and when you pin a layer to warp stabilized footage, the pinned layer will tend to float around because warp stabilizer warps the footage in a non geometric way to try and make it smoother and your composite won't be warped.

There you go. Lots to digest. AE is complicated, slow motion is complicated, all motion stabilizing is complicated and most of the folks that post how to do this in AE tutorials on YouTube are amateurs that don't really know what they are doing or how the software works.