Are their any soy products in your diet? Soy milk, tofu, etc. If so that could be a culprit.
There is an acid in unfermented soy that prevents the uptake of minerals through the intestines. The body only absorbs nutrients to the degree that minerals are present.
Are you buying organic foods or are you getting them from the usual grocery store?
I've found there to be very little nutrition in grocery store foods.
To be completely vegetarian I would recommend growing and juicing your own wheatgrass.
That should ensure a healthy diet.