What's wrong with the prefetching? I'm currently messing around with camera tracking and I've noticed a strange issue. Whenever I prefetch the footage, it works, but from frame 129-160, the blue line isn't there. On playback, once the footage reaches frame 129, it pauses briefly, prefetches those frames as it plays, and from frame 0-47, the blue line disappears instead. And then everything resets and repeats when it reloops the footage.

I'm not sure what's happened, but is there any way to fix this? I've attached my save.
You probably need to set a higher cache limit in your preferences. It is under the System tab. The default is 4 GB which may not be enough for what you're doing.