How many times have they been assist/force fed now? Sometimes it can take up to 10 tries. How much do they weight? How old? Maybe they're simply too weak?

Is everything else normal? No wheezing, pee and poop regularly? Have you taken them to the vet at all?

Feed on a 5-7 day schedule, a little extra hunger might just be whats needed to kick off the feeding response.

Throwing all the advice I can.
Hope they start eating. Don't give up just yet!