For that matter, if you're really having trouble and aren't too low on $ to try it, you might build the okapis an exhibit far away from the guests, so they really have complete privacy. Keep a close eye on their needs and their happiness, and drop a zookeeper in their out-of-the-way exhibit whenever any of the above get low or their exhibit needs food or cleaning.
OTOH, it could just be bad luck that they're not breeding in the available time. If that's so, you should just save the game as soon as all your endangered species are provided with suitable exhibits, and keep on re-running it from that point on, until chance dictates that they all breed before time runs out.