I have gone trough this tread with no luck:
I'm working on a project manly with images. Basically keyframing position and scale. At some point, totally random, i'm unable to select my keyframes. Noting changed in comp.
This is what happends. Set scale to 80%, move keframe to the left. Set scale to 100% move keyframe to right. Select first keyframe and move a bit more to left. Select second keyframe and move a bit more to right. Try to select first keyframe, no luck. Try to select second keyframe, no luck. Now i can't select any scale keyframe on that image. Nothing works. If i disable and reable the stop watch, the problem will start again at random point. Need help!
We can't know. You have not even told us what version of AE much less anything about your system, your comp settings or whether you e.g. you use the disk caches or things like time-remapping. You need to provide much more info.