There can be low B12 now commonly because of the overuse of acid blocking drugs which raise the pH of the stomach acid, and
prevent intrinsic factor from working. The B12 has to be broken out of protein sources by the action of acid and enzymes. Since only 2-4mcg a day are provided this way, it is critical for the B12 to not be interfered with.

Then there are the genetic failures to methylate cobalamin to methyl form. (along with folic acid to methylfolate) This is called MTHFR polymorphism, and it appears that the 10% initially thought occurance may be as high as 30% in some populations.
(a specialist physician came on NT a couple of years ago suggesting research pointing in the 30% direction.)

Another spot where failure can occur is at the CSF barrier. There is a gradient from serum to CSF which pumps the B12 into the CSF (cerebrospinal fluid). This may fail. Also if the serum is low, the gradient does not favor methylcobalamin's passage into the brain. Fibromyalgia and MS patients have been tested and found low in B12 in the CSF.

Other drugs will reduce B12 in the body. Metformin for type II diabetes is one, many antivirals, and long term antibiotic treatments like for Lyme, etc.

We have a B12 thread here that I've gathered much data on
supplementation and biochemistry:
http://neurotalk.psychcentral.com/thread85103.html

Here in the US labs still report lows, below 400pg/ml as "normal" and so many people continue to decline even after testing. It is the FIRST question I bring up to new posters here, because of how common this problem has become. 400 is the new cut off here in US, and in Japan it is even a higher number.
There was even an Mystery Diagnosis program on Discovery devoted to this problem. You could probably find it online now and watch.

Some of the more aggressive integrative doctors are now doing serum transcobalamin testing. But some labs don't offer this yet, and it is more expensive, etc. It is supposed to be more accurate. People in the gray area of possible trouble, can also have MMA and/or homocysteine measured and MMA will be high in low B12 states. MMA will reveal a problem when serum levels may be high (in those with the genetic
MTHFR errors), revealing methylation failures.