Duplication issues

I am making a digital version of a Zombie board game for fun, but have run into some issues with duplication of the pieces. I got these scripts online, and have modified them to suit my needs.
There are 3 enemy types, and when you click each button it duplicates a MC off stage and places it in a box for you to click and drag to where you want it to be placed on the board. My issue is, when I spawn some of the first type (walkers), then spawn some of the second type (runners), it removes the walkers spawned from the stage one at a time. The same thing happens with the third type (fatties). If I spawn 5 walkers, 3 runners, and then go to spawn 8 fatties it will remove the previous enemies with each press of the fatty button like so: 1 runner,1 runner,1 runner, 1 walker, 1 walker, 1 walker, 1 walker, 1 walker, then I will have 8 fatties and no other enemies on the board.
Any assistance would be great, my code is as follows:

On the Walker Button:

on (press){

on (release){
    //_root["Walker"+WalkerCount]._x = 1182.4;
    //_root["Walker"+WalkerCount]._y = 63;
    _root.Walker.duplicateMovieClip("Walker"+WalkerCount, WalkerCount);

On the Runner Button:

on (press){

on (release){
    //_root["Runner"+RunnerCount]._x = 1182.4;
    //_root["Runner"+RunnerCount]._y = 63;
    _root.Runner.duplicateMovieClip("Runner"+RunnerCount, RunnerCount);

On the Fatty Button:

on (press){

on (release){
    //_root["Fatty"+FattyCount]._x = 1182.4;
    //_root["Fatty"+FattyCount]._y = 63;
    _root.Fatty.duplicateMovieClip("Fatty"+FattyCount, FattyCount);

And for good measure, my drag and drop code on each of the off stage MCs:

onClipEvent (load) {
trace (this._name);

Also, to note, the WalkerCount, RunnerCount, and FattyCount are globally declared variables on the main timeline:

_global.WalkerCount = 0
_global.RunnerCount = 0
_global.FattyCount = 0

Thanks in advance for any assistance!