There are many reasons why you may have a difficult time resolving a vitamin B12 deficiency. This could be from the form of vitamin b12 you are taking - but there are also less common, lesser known reasons why you may struggle resolving a B12 deficiency.
Another reason why your your B12 deficiency could not be resolving is if your have malabsorption issues, digestive disorders, or you’re taking some sort of stomach acid blocker. In general if you don’t have enough stomach acid then this can significantly impact the absorption of vitamin B12.