There are a myriad of reasons. One of them is that the gut and digestive system is poor at absorbing and utilizing B-12. Some people can absorb it better than others.

Most vitamins and supplements contain cyanocobalamin, an inferior form of B-12 that isn't neuro-active or bioavailable. Methylcobalamin is the preferred form of B. The difference is night and day for me.

I also read that B-12 in your normal diet isn't really enough. Another thing you have to account for is how much of the planet is on an acid reducing medication. Acid blockers make it almost impossible to absorb B-12. I know when I started B injections, it was like getting my life back.

B vitamins are best absorbed through injection, plain and simple.