it may be timing. If you eat a lot of fiberous foods, your stomach is going to pass them, but sometimes the food itself is so small that it can't always be solid.
When I eat steel cut oats (which have good fiber) they NEVER come out solid.
But if I ate veggies/rice/chicken (when i was lifting) they seemed to always come out solid.
So I got to a schedule where I'd have steel cut oats in the morning and get rid of that by mid day. Then i'd eat a full meal which would give me a solid one.
Your answer may be NOT trying to make a solid one. Eat foods that are healthy but don't give you a solid one, and then try a balanced meal with a kind of meat and veggie and see what happens.
Ya catch me?