Have you teased the frog with prey yet? It can be very hard with nightcrawlers. I personally use locusts if one of my frogs is being really stubborn. Just grab one with your tongs, make sure to grab it 2 cm away from the head, still on the hardpart of the locust.
Then wiggle it against it's lower lip. Don't give up, it might take a while. If the locust keeps grabbing onto the frog, cut it's front legs off. (I am not joking.) :P
I've noticed that my tough feeders are easely persuaded by locusts at first, then nightcrawlers.