Breeding fails

Providing both parents are 100 BLUP at the time of breeding, the offspring will be an average of the parents GP plus a small gain (used to be 1.4 GP points years ago). Not sure which mare you're referencing but I just looked at Sheep Stealers' offspring and they all have higher GP then her cuz she was bred to stallions with higher GP.

As for the IBS, if she was bred with tears she may have just gotten the top IBS possible. It can be quite difficult to get top IBS when tear breeding. I've gone 75+ coverings without getting within 50 points of top IBS.

How old is she? Age can affect those things unless they're equipped with a Philosopher's Stone. I can't think of anything else that might be the reason.

The minimum GP gain for each generation is 0.50, regardless of the BLUP, so if the stud has a higher GP than the mare, then the foals should be higher GP than the mare, too. I agree with Crystal Coven about the IBS, though, perhaps she hit the IBS lottery at birth, but her foals weren't as lucky. The IBS threshold is large enough that even the GP gain won't help, especially if both parents are top horses.

sorry, but this is incorrect. Only the GP and the BLUP of the parents affects the GP and IBS of the offspring. There is a +/-4% variance though, this is possibly what the problem is.