Hi there. First of all, there are three types of Boolean operations, so I assume you are doing the Boolean difference operation.

With the difference operation, order selection is VERY IMPORTANT.

The first object you select will be the one you take the "bite out of". The second will be the "shape of the bite". When you have selected the two objects, the color of the selection of the first one will be white, and the second will be green.

My advice is to try to reverse the order of your selection, and try again.

Hope that helps.
Drought