swap symbol not working

Hello-
I have an fla file where i was swapping instances of bitmapped symbols no problem, and then for no reason i can tell, it just stopped working. Ive been trying to test how & why this happens, and the only thing I can notice is this:
I'm starting out with a bitmap that has been converted to a symbol. I am motion tweening this. when I motion tween from the first to the 2nd keyframe, I can swap the symbol just fine on the 2nd keyframe. When I add a 3rd keyframe, so that the 2nd keyframe has motion tween coming in and going out, the 2nd keyframe loses the little black edge (not quite a box, since it was only on one side). It still has the black dot, so I think it's still a keyframe, but after that, i can't swap a symbol instance on the frame any longer. likewise, if I insert a 3rd frame between two that already have a motion tween, I can't do a symbol swap on that third frame. I select swap, the dialog comes up, I select the new symbol, and NOTHING HAPPENS. Swap symbol does not update the frame.
I know it's possible to have motion tweens in and out of a frame that contains a symbol swap as i've done this many times. maybe this file is corrupt or there's some setting that is causing this. Anyone have a clue? Using mx 2004 pro.
thanks
bc

here's someting else I noticed-
in an older file where the motion tween/swap combo is working, the frames are grey, with a little black edge before each keyframe. Where the motion tween/swap combo isn't working, the frames are light blue and there is no "box" around the keyframes. as far as I know, I did the same thing in both cases. How can I make the blue unboxed keyframes into grey boxed ones, and what does this mean anyway??
thanks much if anyone can help!!
bc

Having the same problem as you.. and I'm using CS4.. Don't post on this forum NO ONE uses it :/

Remove tweens, swap symbols, and recreate tweens. Tweens work fine only between keyframes containing the same symbol. You might need to add extra keyframes just to have the tweens work well.