Are you taking cyanocobalamin or methylcobalamin?

When using oral you should take on an empty stomach as food will block absorption.

Here is the B12 informational thread with medical citations:
http://neurotalk.psychcentral.com/thread85103.html

Low end of the old normal range is at or below 200.
The new cut off for normal is 400 (US units)
Methylcobalamin is best now, because the cyano form has to be converted to the methyl anyway. If you have a genetic error in this conversion (which is pretty common--- 10-30% of people have this called the MTHFR polymorphism), you cannot make methyl form from cyano and it won't work. Methyl form is very inexpensive and affordable and available online at many outlets today.

Since only about 10mcg is absorbed from 1000mcg oral, it will take a while to get up to a better level. Hence the higher dose will work better and faster. But that is your choice after all.
The oral absorption only really works well on an empty stomach as well, so make sure you do that at least.

We see so many people here who are not being given adequate treatment for B12 deficiencies. So if you know all this information already, please excuse me, but a large number of those coming here are either never tested, and/or never treated properly. Many doctors don't even know what methylcobalamin is (the biologically active form for B12).