J.
Hi,
I know just what your going through. My son will be 4 in Feb and just started pooping on the potty 2 mos ago.(I thought I would never see the day!) He too was fully potty trained and would only poop in a diaper. Long story short, I found out he was afraid of going on the potty because he associated being constipated with going on the potty somehow. He would poop about once every 5 days. Well I bought fiber sure-it's a white powder that you can mix into liquids or foods and it is tasteless, and odorless. So everyday I put 1 teaspoon in a sippy cup of water and stir. It's amazing to see how it clouds up for a minute and then gets clear again, and my son drinks it all down and has no clue! But it works great after a few weeks of being on it he poops regularly almost every day and for him he needed to see that his poop wouldn't hurt anymore now that he is on a regular schedule with pooping everyday and also having it be soft and not rock hard anymore, then he went on the potty for me like a charm! We're still using fiber sure everyday.
Good luck to you