How long have you had the cardinal? They can be rather shy fish but after acclimating to it's surrounding they will most often do quite well.
Is this still the only fish in the 29 gal
and are all the levels still good?
Brine shrimp can often be a great enticement for new/finiky eaters as well as a few drops of garlic extract just before feeding. Although brine is not a very nutritious food, the important thing is to get the cardinal to eat. Once accomplished you can wein it over to a more balanced diet later. Make sure you do not overfeed in hopes of enticing it further or you will risk water quality.