@Truc
It does take time to gain weight back, but the fact that you keep losing weight and hair is not typical. It might be good for you to consult a dietician with j-pouch experience who may be able to guide you about what to eat.
In the meantime chewable multivitamins like Flintstones brand would be a good idea, especially if you are still on a low fiber diet. I'll list some of the things that helped me to gradually return to my pre-illness/ pre-surgery weight. Others may have different suggestions.
It's important to get enough protein. I ate lots of roast chicken. Pork cooked until it fell apart. Eggs. Plain Greek yogurt. Cottage cheese. I learned to stay away from sugar, white flour, pasta that can be hard to digest. I picked vegetables carefully and ate them sparingly. I recall asparagus tips, very soft green beans, and baked potatoes were good for me. And small amounts of acorn squash also ok. And cooked spinach - also very small amounts.
My dietician suggested I make my own shakes with unflavored whey protein powder. I used bananas, peanut or almond butter, nut or cow milk with Jarrow brand protein powder. They were delicious, and the hardest thing was not drinking too fast.
i also made rice pudding with almond or cow milk or a combination, but very little sugar. I digested it well and it added calories.
The other thing that worked for me early on was cream of wheat cereal which is enriched with vitamins calcium and iron and easy to digest. A little banana added to sweeten it. And oatmeal - quick cooking variety - cooked longer than the instructions say and with lots of liquid so it was very soft- really helped with output consistency and frequency.