Sometimes my pacman would not eat when I uncovered him, so I just simply and gently pushed a little substrate back on top of him to cover him up, but leaving his eyes uncovered and then take the food back out. If the Pacman frog feels comfortable, they will go and get food when it gets close to their face. If a worm or cricket is literally crawling on top of their face and the frog is not reacting, they are most likely not interested, as they would've taken a bite already. (Or in my case most of the time, still trying to swallow the last one).