HELP! my zebra finches keep dying!?

on the 15th of this month i bought two zebra finches, a male and a female. The cage is your standard 15'' wire variety. a few toys, a nest, perch, fresh bottled water, and a separate water dish for bathing. millet and gravel are also included. their cage is high up away from any disturbances or drafts. i had the finches for four days, and they suddenly died! we decided to replace them, and again, four days later they died...we have no idea what's going on...can anyone help?