So the problem clips had speed changes applied to them originally?
If so, the best practice is to bake the speed changes into a clip. (Export the speed change as a single self contained clip and then put it on top of or replace the original clips.)
It’s really easy to mess up a clip with a speed change on it. Plus, if you’re going to be using Color for color correction, you’ll want to have the speed changes baked in as that’s one more step that can totally change the timing of your speed adjusted clips.
-Russ