It can take a while 6 to 8 weeks in some cases, the problem is that it is a viscous circle, the later they start eating the less they really want to eat, so getting them started quickly is really important
Generally speaking if something failed 2 weeks in a row you want to try another prey item.
The best prey items for an hatching to get started are hopper mice, you get a much higher success rate when using hoppers as first meal and switching thereafter.
With hoppers my success rate of hatchlings eating first time offered is about 98%