Flash has it set so you either have a different layer for each graphic tweening (for example, one layer dedicated to tweening the arm, another for tweening the leg) or you have a blank frame in between each tween to make it work.
For example, if you wanted to tween a gun and then on the same layer tween a different graphic like a bodypart. Create 2 keyframes, join the tween for the gun, then create a blank keyframe, and then add the different graphic and then tween that.
If you don't create a blank keyframe inbetween, it will think the new graphic is the old one.
I hope this makes sense, and i hope this is the problem you were referring to.