break apart clip items from synchronized clip?

I've created some synchronized clips containing audio and clips from 3 cameras. I need to break apart them on the timeline but the command appears grey on the menu. I remember that I could do this on previous versions (now on 10.2.1). What is even more strange is that if I just copy the clips from the synchronized one and then paste them on the timeline, they loose the perfect match of sync. Two clips are from canons on 23.98 fps and the third one is from Panasonic gh4 at 24fps. Could this difference between framerates be the issue?
Many thanks

Pablo

Just to be sure, Pablo, you are not mixing up synchronized clips and compound clips?

No, but if I delete the 4k footage from Panasonic, then I can break apart the items. Synchronized clip function may force the 4k clip to be 23.98 rather than its native 24 fps. When it is inside the sync clip, it is really in syn with the others clips, but if I copy all the items and the paste them in the main timeline, the 4k footage loose sync. At the beggining it seems Ok but after some second it starts gradually loosing the proper sync. Must be a fps issue I guess.