If topical ointments (such as A & D) don't work, you may want to ask your pediatrician.
Other than that, one of the best cures for diaper rash is air. Let the baby go bare bottomed as much as possible (place waterproof pads underneath the baby when s/he is on something difficult to clean).
You can also try using a blow dryer. Turn it to the lowest possible setting and aim it so that the air current blows across the baby's skin, not directly at it. Be sure not to let the dryer come in contact with the baby (and limit the use time) so as to prevent burning baby's tender skin.