Where does your followthrough end up? Your drawing hand should end up by your ear, with your fingers possibly touching your shoulder. If your drawing hand is ending up lower than that, you could be pulling down slightly when you release the arrow, lowering the nock end of the arrow and causing it to leave the bow at a slight upward angle. Try holding your drawing elbow a little higher, so it feels like you're pulling back at a slightly upward angle. Actually, you'll be pulling straight back, but it might feel like an upward pull at first if you're used to the feel of a downward pull.
Otherwise, if you're happy with your form, I know Rick is a great believer in observing your sight picture. Just observe whatever sight picture makes the arrow go high at 25 yards and make that your 30 yard sight picture instead.