When in Fullscreen OSX Yosemite won't show the dock if "auto-hide-dock" is enabled.
I was a bit stumped by this, but actually I think there is an explicit procedure. 

When in full-screen mode, move the cursor to the bottom of the screen; then drag down with one finger. The dock will then appear. 

It appears it doesn't matter where you do this (which application icon you happen to hovering over) as far as I can see.