Elementary Question about Masks and Transparency

Simple Question.
For some reason I forgot how to perform this action... I have a sprite which is a circle and is transparent inside (circle inside is transparent)..... Objects are inside the circle in the transparent area... however when I add a collision event with the object wihitin a circle it doesn't move even though its programmed to move. How do I modify the Mask so the objects inside the circle can move... Objects inside circle are programmed to move at random directions