How many have you tried? I have had the best luck starting mine on Zupreem Fruity and then going to Zupreem Natural. It took my quaker about 2-4 months to start eating pellets regularly, and another 6-8 to get off of the fruity, so don't give up! To convert him, I provided pellets 24/7, and seeds twice a day for 15 mintues. I gradually reduced the amount of time seeds were available until it was only once a day for a two minutes.
My other quaker I converted by feeding Harrison's bird bread.