Premiere Pro Speed/Duration Not Working On Timeline

Hi there,

I'm trying to make 60fps footage slow down on the timeline using the speed/duration panel by changing the speed from 100% to 40%, but it is choppy. Yes, the footage is shot at the correct fps, shutter speed, so that's not the problem. 

If I click the original video source in the project bin, select modify, and click interpret footage from 60fps to 24fps, the video is perfectly slow motion. So, why does it not work on my timeline?? 

I've edited directly next to someone using the exact same footage, sequences set to 24fps, and when they make it 40% it works, but mine never works. Please, someone explain why slow motion doesn't work for me, specifically on my timeline. It doesn't make sense.

Thank you in advance!