does anyone ever have trouble training your pets? not really the training to every command really, but the simple things like house breaking them?

right now, i'm have trouble re-training two of my cats to use the litter box. a few months ago, we had a kitten who wouldn't use the litter box. so, she was left outside with the feral cats (it wasn't bad since one was her mother). now, she uses the litter box with no problems. but now, my two older cats (one is almost 3, the other is a year and a half) won't use the litter box any more. the younger of the two goes poo right outside the box, the other, she keeps peeing on the baby's changing table. i've been trying to get them to use the box, but it isn't going well right now. i think the reason they're doing this tho is cuz they're jealous of the new baby.

so, do any of you have problems like this? do you have any suggestions of what to do about problems like this?