I second getting the vet out. If she was bred year before last and no foal then possibly something wrong with either her or the stallion. Was she bred three cycles in a row or three times in what was one cycle? Could be timing is off. Not continuing to cycle does not necessarily mean bred. She also could have taken and slipped the foal.
Either way if she is in foal now is the time to be feeding for that foal.
Some horse people change their horse, they change their tack and discipline, they change their instructor; they never change themselves.