I could say to go through the fresh kill and gradually increase the time since the kill, but I'm more inclined to advise that you just let it get hungry. If it has good body weight, there would be no harm in letting it go a month (or more).
One question first, is it being feed in an area and at a time of day where is feels secure? It might be taking the live prey because it's still there when it's ready to eat - as opposed to you pulling the food out after an unsuccessful attempt. If you have not already done so, let the caiman get good and hungry and then try putting the food at the waters edge at night. Then turn out the lights in both the tank and the room and just wait it out for a day and see what happens.
Specs are usually good feeders so this one should come around.
Bill