Weight gain or loss has to do with metabolism and nothing to do with the J Pouch. I have struggled to keep weight off since day 1 with the Pouch. My surgeon refused to operate on me until I got down to 160 pounds and I have not been anywhere near 160 ever since. I hover around 190 these days which is probably 25 pounds too heavy.
I wouldn’t worry about it at all especially if you are eating cautiously and not a full diet yet.
Once you are “out of the woods” on the blockage threat and eating a full diet, there are various strategies that can be executed to stimulate weight gain if you are still struggling to put on weight. For now, I would be more focused on eating easily digestible foods, in small quantities, and consuming maybe 5-6 small meals per day.