SOLVED!
The Problem lied within the layers.
The Parent clip had 4 layers(bottom up); Background, Water Movement Background, MovieClip-Overview, Water Movement Foreground.
I removed the Top Layer: Water Movement Foreground and tested...It worked...however, when I added the same movie clip of Water Movement Foreground to show OVER the rest of the movie clip (overview).
However, when I moved the Button: Replay_btn and Inside_btn, above all except the Actionscript frame layer...it worked!
So, if your buttons are not working, but you have no Compiler or Output errors, ensure that there are no other Frame Layers ABOVE your button set.