Try jigging the food item a bit on some tweezers. Sometimes they need to see the food. I've noticed if I give Gerald an earthworm and it falls into the tank too close to him, it's like as though he can't see it moving but he has a little bit of a fright when he can feel something moving near him. So now I dangle it in-front of him rather than drop it in the tank.
Use plastic feeding tweezers if you're afraid at first, be careful though, they will either bite the food item, the tweezers or your finger... I wouldn't worry about the biting bit, majority of the time its the sticky tongue that gets stuck to you!!