Help with break pads//car breaks?

ok so i drive a 2000 cadillac deville and the break pads ran out 2 months ago so i got them changed now 2 months later now there starting to squeak again??? so i thought it was the break pads again so i had them checked at BIG O TIRES and he told me the pads were fine but the rotor builds up some time of slime and that it will sqeek till the pads are broken in he said just to wash it with water and i did but it still squeeks when i press on the breaks any help???should i clean it with break cleaner or something any advise would help me thanks