Quakers can be very territorial around the birds cages. Its an instinctual behavior and to some degree will always be there, especially during breeding season. It sounds like you have a male/female pair, which makes the inclination to bite even stronger in your bird. First, take a look at how you are going about this. Are you reaching in respectfully and slowly so as not to startle them? If not try changing your approach.
I know my quaker will sometimes nip at me no matter how carefully I proceed into the cage. She used to downright bite me but weve gotten it to a warning peck on occasion now. Since my quaker is very food motivated, I started reaching in the cage with a favorite treat.