dropTarget on overlap layers of movie clip

I created a movie clip, a_mc where there are many other layers of movie clips in it with its own instance name for some other purpose. Then, I created another b_mc where I insert the dropTarget script in it. What i want is, whenever i drag b over a_mc, the c_mc will go to frame 5.

Everything is working now except that whenever i drag b_mc over a_mc, only certain parts of a_mc works. I guess it’s because of the many layers in it.

Can someone please help me?

This is my script in b_mc:

on (press) {
this.startDrag();
}

on (release) {
this.stopDrag();
if (this._droptarget eq “/a_mc”){

tellTarget ("_root.c_mc")
gotoAndPlay(5);
}
}

Do i need to add some other script in b_mc for it to works on every parts of a_mc?