Yes Perfecto and stwert are correct. Booleans are VERY tempermental mate...and frustrating. I have eve had to extract the faces in the area I want to boolean, then do the union and then snap the extracted faces back to the original geo. Can you screenshot what you are trying to do to give us a better idea? There are many ways to do things in Maya..you might be able to do it as stwert has suggested by manually joining them...sometimes its less of a pain to do it that way.
"A Darkness at Sethanon", a book I aspire to model some of the charcters and scenes