One more thing- when you're right there feeding from tongs & they grab it, DO NOT MOVE. That might be the problem- that bit of visual distraction, which in the wild could be a predator coming after them while they're busy with food.
How long you wait- how many times you try- is up to you. Not everyone does the same thing- I don't think I'd reheat more than 3 times, & if they don't take it, personally I'd leave it in the enclosure overnight- it might be they'll take it when lights are out & they aren't being watched. In fact, the last re-heat would be a good time to darken the room & just leave. I hope you don't have a personal life too?(You could also put a dark covering over their home, so they're not aware of you. And no peeking, lol. Seriously, they might see that tiny bit of light & motion-)