what is "30-50 XSP/p" and your idea of fairly regularly?

ime, most hogs that are fed properly sized prey 2x a week, grow well. that is providing they have proper husbandry.

i have one that did not eat regularly until she was 6 months old. now she eats 2x a week like clockwork. i have dealt with several that were "maintenance fed" by the breeders, and have stunted growth. i keep very accurate records, and their growth rate is not even close to ones that have been fed 2x a week since hatching.

although... even with the slow growers, or ones i consider stunted from maintenance feeding, they still have gain much more than 6g in 2 yrs.

so, i would be checking your husbandry... ambient temps, hot spot, hides, water, and feeding 2x a week.