Try tong feeding the frogs with small worms, I do not place food in my Xenopus tank to feed my guys as the females tend to be very greedy and eat all the food, I have one who is so greedy she has nearly killed herself with overeating twice, so I always individually feed my frogs ,that way I can ensure they all get fed. This also has the added advantage that the frogs associate you with food and dont hide when you approach the tank.