To assign code to an object the object has to be present. If you move your
button functions to the first frame where each appears it will work...
I know, it's enough to drive you up a tree, thru the roof, and elsewhere.
One other thing, you really only need one invisible button in the library...
you can name it "invisible_btn". The same one can be used for all of them.
You just take that same button and stretch it as needed for each use, assigning
each a unique instance name. This will help reduce the file size.
Also, I can't tell for sure what you did for the fade to black transition, but
that could be simplified as well... there are alot of graphic symbols that are
likely produced from whatever you did. They could all be eliminated, likely
reducing your file size even more..