What can cause vitamin b12 malabsorption? especially if you have low value, <109 pmol/l?

Intestine issue: B12 is absorbed in the far end of the small intestine. Conditions that involve that area can reduce absorption (infection/surgery/etc,). Dietary habits that reduce intake (vegan only) can lead to low levels.

Multiple causes: Gastric atrophy and lack of intrinsic factor, terminal small intestine disorders, fish tapeworm infestation are the usual suspects. Take sublingual vitamin B12 and if that does not help take B12 injections.