Several different things can cause this. I hope you’re being given a customized prep for the pouchoscopy - I’d expect something like numerous small-volume tap water enemas rather than anything from above. Some of the potential causes aren’t visible on pouchoscopy. For example, a pouch prolapse is best diagnosed with defecography. An overly tight anal sphincter is diagnosed with anal manometry, though if you can’t pass a Fleet’s enema bottle, or your doctor can’t pass a scope then that says a lot. Some people accumulate solidified psyllium or stool in their pouches, which is more like constipation. A stricture should be visible on pouchoscopy, though a pouchogram might be more revealing.
Good luck!