Gravity... again

ok I have a puzzle.....
I have three columns with actors. Every column 8 actors. The actors are touching each other (they had to because I will use COLLISION DETECTION). They have ACCELERATE TOWARDS 270 and 500 because when i touch one actor it destroys and i want the above actors to fill the blank position (I do not want SCENE GRAVITY). They are MOVABLE. BUT... when I touch and destroy an actor, they fill the blank positions... sometimes!!! Some are stuck and have to destroy other actors so to move down. Any suggestions???
I have three columns with actors. Every column 8 actors. The actors are touching each other (they had to because I will use COLLISION DETECTION). They have ACCELERATE TOWARDS 270 and 500 because when i touch one actor it destroys and i want the above actors to fill the blank position (I do not want SCENE GRAVITY). They are MOVABLE. BUT... when I touch and destroy an actor, they fill the blank positions... sometimes!!! Some are stuck and have to destroy other actors so to move down. Any suggestions???