During the following months, I'll list at least 50 possible ways you could develop B12 deficiency. Many points on the list would be interesting for non-vegans as well, but (of course) I will focus on B12 issues that are relevant for plant eaters. You're all welcome to discuss each and every point on the list, but please do it by starting a separate thread.
As a reference, the late dr. Victor Herbert insisted that nobody would need more than 0.1 mcg B12 daily (the B12/homocysteine issue was probably NOT taken into consideration here), and he wrote that "0.2-0.25 micrograms per day absorbed from food is adequate for normal people". The Recommended Daily Average is 1.0 mcg/daily according to World Health Organization, USA recommends 2.4 mcg daily. Safety margins are normally built into RDA values.
1) You might be eating only B12 poor plants.
According to this study, which focused on spinach, barley and soy only, 1kg spinach grown in maneured soil contains 17.8 mcg B12 (dry weight), while soybeans grown with manure had only 2.9 mcg of B12 (and only 1.6 mcg with chemical fertilizers.) Whether plants are a reliable B12 source or not is a hot topic, but this example should document that the B12 levels should NOT expect that all plants contain enough B12, even if they are organically grown, raw, or eaten unwashed.
Some plants contain B12. Some of these B12-molecules are so called 'B12 analogues', which may - or may not - be a problem (this is another hot topic). Whatever you do, and whether you are a vegan or not, we recommend everybody to pay attention to their B12 levels.
If you want to get B12 from plants, at least make sure you eat plants that may contain B12.