Ooh. There is a thing called pernicious anaemia where you can't absorb vitamin B12 properly so only a fraction of what you eat gets in. It's a thing that just happens to people. My grandmother had it. Supplements might have been too compact for much of the contents of the tablet to get absorbed. It is worth getting tested...they do one blood test and tell you a couple of weeks after, I had it done because it's a little bit genetic.