Sometimes it can also be the lack of another nutrient needed as a co-factor. Potassium is needed for B12 utilization (and i think by a lot of other things, as well). B2 is needed for a lot of things including iron. Just taking more of the thing you are deficient in may not be the answer.
Agreed. And in a lot of cases, when a mineral shows up low in a hair test there may be plenty of it available, just not those other factors to get it into the tissues.
My regime is quite comprehensive, and timed to avoid as much competition for absorption as could reasonably be avoided. And where appropriate, I've kept the doses of certain things quite small, so as not to overstrain stores of others (e.g. B2 is often a limiting factor since only so much of it can be absorbed). I wouldn't be surprised to find out that I was having absorption issues due to gut problems, though. Will keep experimenting. It's always an ongoing process.