Hi, well it be could lack of digestive enzymes, maybe stomach acid and because of that, there can be SIBO and/or some fungal issue (like Candida). This is according to my experience.
If this situaution is going on for years, there could be lack of minerals and vitamins. For example - my blood magnesium levels are always low normal, zinc also, copper and I guess B vitamins are problematic but don't know where to check it.
There is maybe liver (bile) situation, but it's a guess.
All this is according to my experience and/or testing
Magnesium, zinc and B vitamins are included in few hundred of enzymatic reactions in our body including liver detox phases and if they are low - it's not good situation.