Just wondering about a few things if I may -
Are you vegan/vegetarian?
Have you been tested for intrinsic factor binding and blocking antibodies? And parietal cell antibodies? Also folic acid?
Folic acid and B12 function together synergistically in the body.
What type of B12 supplement is it? Sublingual? Have you tried injections?
Sorry about all the questions, just want to be thorough in getting the information to gain a fuller picture so I can help you.
I'm also curious about digestive function. Though you don't need to divulge here, unless you are comfortable, just observe how well you digest food, as your intestinal flora plays a vital role in B12 production and absorption, regardless of intrinsic factor levels. Overgrowth of yeast (candida, clostridia) will also affect levels of B12. The liver is also heavily involved in B12 production, so anything that effects the liver like smoking, alcohol, caffeine, medications etc can affect B12 levels. Medications like anti-epileptics, antibiotics, hormone therapy can reduce levels.
Hope this gives you some insight and if you can provide more information I would be happy to help further. Cheers, Helen :)