I don't think there is one answer however we have tried about every trick in the book and we are succeeding. Our grandson is 5 yrs. old and only the past few months have we reached success whatever that may be. I found a site where a woman shared a social story we made up for her son. I printed it out and got all kinds of pictures and we read it over and over. It was fabulous. He began to understand the concept. However we have learned do and don'ts.. We cannot use public restrooms that have automatic flushes.. He has to hold his ears during the flush so we give notice "going to flush now, hold your ears." We have given him an ipad to sit and play all kinds of games while waiting for the urge. and recently he asked me if I could sing the poopie song to help the poopie come out... So I make up a song and sing away... Come out Poopie Come out Poopie we're waiting here we're waiting here I can harldy wait to see you I can hardly wait to see you come right now... Tune from a kids song... or whatever... HE yells Nana it worked.... Just we willing to try anything crazy that brings success to that special person....

There's a really good chapter on potty training in the book "A work in Progress".

Find something your child loves and make it a reward for his/her potty training efforts. Praise your child no matter how small or big the step is within your potty training experience

Yes, I am experiencing the same issue but help is on the way. Purchase a few pairs of underwear and allow him to only wear those while at home. Try taking him to the potty every 50 minutes even if her complains.
Make potty time fun, stickers, books, and even a small mirror to hold onto to make silly faces. Always cheer and do a potty dance each time he is successful. And for those times when he's not just say, O.K. we need to poop and pepe in the potty. Save the pull ups for nighttime and for going on those long outing. He will get there. My child is showing improvement each day. Be consistent and remember every 50 minutes. Good luck! Also find pictures of his favorite character and let them go to the potty with him.

Try the price chart every time he attempts to use the toilet give him a sticker after a a full week of stickers give him a treat.. To help motivate him