Once your body stops absorbing B-12 you no longer are able to metabolise it. Which means simply that your body will not retain it. It is only present in cases like Pernicious Anemia. I have it so I know first hand. You can ask for a simple blood test to test the acid level in your stomach and there are other tests that will diagnose pernicious anemia. Determine that first. You get the shot and your body usues it all up. So in some case you may need the shot more often. I get mine every 2 weeks. My doctor has run tests to prove that this will be sufficient. It's simple frequent blood tests every few months on different sets of shots to show where you are at and what your body will retain and function on. Put your shots up to 2x a month and retest after 3 months. If you b-12 goes up great. If not you made need it more frequently. Please also note that there could be an underlying reason that could cause the b-12 to drop which resulted in the pernicious anemia if that is what you have. You should get an endoscopy to rule out things like parasites, bacteria, chrons, celiac disease or stomach tumors or cancer. You could also be losing blood somewhere in your body or you may just need the shots more often than the routine once a month. Find a good doctor and talk about the causes and run a series of tests to rule out and then talk about increasing the b-12 shots. Good luck and God bless.
Go to www.wrongdiagnosis.com under the forums and click on the one that says "Vitamin B 12 deficiency is commonly misdiagnosed" by Sally Pacholok RN who also co-wrote the book Could It Be B12?. She is the most informed and experienced person to ask about B-12 deficiency and she will answer you back within a day or too. Her book is amazing and very educational for anyone who thinks they may have a B-12 deficiency (and for many who have been misdiagnosed with something else!) Hope this helps!