I have the same issue for probably two years now. I'm having my j-pouch for 20 years this year. During that time I actually had only two examinations done, the last was at the beginning of this year, a few month after the first time this evacuation issue happened. They didn't find anything that could explain why I felt this constipation thing. The only result was that there were a "few minor hemmorhoids". Nothing serious.
When I feel the urge to use the bathroom it often happens, that only little amounts of stool comes out. Sometimes the pushing seems to even close the "door" and I feel as if there is a pressure valve somewhere that is shutting down the more pressure I use. A few moments later after leaving the loo I again feel constipated and the whole thing starts over again.
Only when I'm able to release a portion of gas together with a "plug" of stool, I get rid of the feeling of constipation.
Eating spaghetti (or worse) chocolate makes it even worse.
I haven't found a solution to this problem yet.