Bath & body products are complicated things. A body wash can contain anionic surfactants, amphoteric surfactants, cationic polymers, hydrolyzed proteins, esters, thickeners, preservatives, fragrance, and so on. Even within that list you see different types of surfactants - sulfates, sulfonates, sulfosuccinates, glucosides, etc. How do you figure out amongst that giant list that one thing you shouldn't use?
Before I made my own products, every single thing I used had to be "sensitive skin, unfragranced" type stuff. Now I can use anything I want, including fragrance oils! (Yay!) What changed? What within those store bought products bothered my skin? I've never used SLS in any of my products - it is one of the least compatible with skin (meaning, more irritating) than the surfactants I use now - so I could extrapolate and say it was SLS, but I can't say for sure. Or could it be the lack of moisturizing ingredients? Or the type of preservative? With a list of ingredients a paragraph long, it's hard to figure out exactly what annoyed my skin so much.
I found out my hair didn't like silk by adding it a conditioner recipe that worked well for my hair. I changed one thing - 2% silk - so it seems obvious that was the culprit! But really, I could have put a tiny bit more BTMS or a little less cetac or a tiny bit more preservative in the mix and that could have caused the problem. (I have tried it a few times and I frizz out every time, so trial and error worked in this case...)
Just curious how you found out a certain ingredient didn't work for you!