Drag and Drop, unable one object to be droped on target if other is already in there.

the first problems i see are you've nested named functions and you're using an enterframe loop incorrectly.

unnest all named functions.

remove your enterframe listener unless it's needed for something and, in that case, only include what's needed in the listener function.  that would look more like:

piece001_mc.buttonMode = true;

piece001_mc.addEventListener(MouseEvent.MOUSE_DOWN, fl_ClickToDrag_001);

piece002_mc.buttonMode = true;

piece002_mc.addEventListener(MouseEvent.MOUSE_DOWN, fl_ClickToDrag_002);

function fl_ClickToDrag_001(event: MouseEvent): void {

    piece001_mc.startDrag();

    stage.addEventListener(MouseEvent.MOUSE_UP, fl_ReleaseToDrop_001);

}

function fl_ReleaseToDrop_001(event: MouseEvent): void {

    piece001_mc.stopDrag();

    stage.removeEventListener(MouseEvent.MOUSE_UP, fl_ReleaseToDrop_001);

    if (targetA_mc.hitTestObject(piece001_mc.tar001_mc)) {

        piece001_mc.x = target001_mc.x;

        piece001_mc.y = target001_mc.y;

        //play sound correct answer

        channel1 = mySound.play();

        channel1.soundTransform = volumeAdjust;

    } else {

        piece001_mc.x = else001.x;

        piece001_mc.y = else001.y;

        //play sound wrong answer

        channel2 = mySound2.play();

        channel2.soundTransform = volumeAdjust2;

    }

    checkForEndF();

}

function fl_ClickToDrag_002(event: MouseEvent): void {

    piece002_mc.startDrag();

    stage.addEventListener(MouseEvent.MOUSE_UP, fl_ReleaseToDrop_002);

}

function fl_ReleaseToDrop_002(event: MouseEvent): void {

    piece002_mc.stopDrag();

    stage.removeEventListener(MouseEvent.MOUSE_UP, fl_ReleaseToDrop_002)

    if (targetA_mc.hitTestObject(piece002_mc.tar002_mc)) {

        piece002_mc.x = target002_mc.x;

        piece002_mc.y = target002_mc.y;

        //play sound correct answe

        channel1 = mySound.play();

        channel1.soundTransform = volumeAdjust;

    } else {

        piece002_mc.x = else002.x;

        piece002_mc.y = else002.y;

        //play sound wrong answer

        channel2 = mySound2.play();

        channel2.soundTransform = volumeAdjust2;

    }

    checkForEndF();

}

function checkForEndF(): void {

    if (piece001_mc.x == target001_mc.x && piece002_mc.x == target002_mc.x) {

        p6.visible = true;

        sig7.visible = true;

    } else {

        p6.visible = false;

        sig7.visible = false;

    }

}