Potty training Autism

My daughter Amani is 4 yrs old now and a twin as some may know already, and I have been having some trouble with potty training her. The problem is really teaching her how to poop in the toilet. It has been very frustrating for me especially that she'll be starting kindergarten this September. She is able to pee in the potty but refuses to poop in it. I am looking for an explanation and solution. If anyone has any suggestions please post them in comments. If any one is having the same problem with their child please send a shout out in comments and let me know if their is something that you are currently trying with your child. This is a desperate cry for help!!