B12 Deficiency and digestive problem

Patient: I’ve had a b12 and folate deficiency and got b12 and folate injections twice a week over a month after that my b12 levels were at about 1700 ng/l. But after two weeks without injections my levels were at 500 ng/l. Now i take b12 pills (cyanocobalaminum) every day and they stay at about 500. But if i don’t take them my b12 drops in 2 days from 500 to 400 ng/l. Does that mean i can absorb the b12 but can’t store it in the liver? And why could that be? (I’ve had a gastroscopy and a colonoscopy without any results)

Symptoms: Slow digestion, stomach and small intestine pain in the nights