1180: Call to a possibly undefined method... Possibly?

Question

I have buttons, that when clicked will create an instance in one of 3 movie clips within a movie clip.
For instance, this works just fine:
b_IF_4G5_TOP.addEventListener(MouseEvent.CLICK, b_IF_4G5_TOP_add);
function b_IF_4G5_TOP_add(event:MouseEvent):void
var b:MovieClip = new IF_4G5_TOP();
b.name = "IF_4G5_TOP";
layout.substrate.addChild(b);
but this does not:
p_conn_h.addEventListener(MouseEvent.CLICK, p_conn_h_add);
function p_conn_h_add(event:MouseEvent):void
var b:MovieClip = new PC_H(); <- This line produces this error: 1180: Call to a possibly undefined method PC_H.
b.name = "p_conn";
layout.connectors.addChild(b);
Which is odd, because as you can see in the first example, I have not "defined" var b:MovieClip = new IF_4G5_TOP();
Both objects exist in the library, are movie clips, both targets and objects are accuratley named and exist.
Am I missing something obvious? Thanks in advance for your help.

Answer 1

Thanks for the suggestions, guys.
I did try renaming the movieclips, no success.
The movieclips that work fine, like IF_4G5_TOP have no as linkage or anything. The code simply places a copy of the movieclip from the library onto the stage within its target movieclip.
I even tried to change their target to the same target as the ones that work:
layout.connectors.addChild(b);
to
layout.substrate.addChild(b);
But no go...
Why do you suppose an identical function would work in one case, but throw errors in another? It's seemingly irrational inconsistencies like this that are so frustrating about actionscript.
Any ideas? Thanks again.