3 year old won’t poop on the potty

My son turned 3 in July and although he will willingly pee on his potty, he refuses to poo on the potty… he has pooped on the potty twice before. And he has tried again recently but it almost seems like he can’t get in the position he feels comfortable pooping in? He bent himself over and tried and tried no success. I have no idea why he would feel anti potty after going successfully twice before, but no bribing works. No candy, toys, whatever will work. Please someone give any advice on how to move past this. I feel like we are behind compared to everyone else with the potty training journey.