I'm not an expert but I did figure this one out. Even though you may test low for B12 it's not a B12 deficiency if adding B12 doesn't help. It's a FOLATE deficiency. I don't understand the chemistry but you can try reading up on the web. So, get some methyl folate and take at least 400 mcg four times a day. Don't take in the evening or you won't sleep.
Apparently this is a common problem that not many Doctors understand. DO NOT take folic acid as the human body cannot utilize it. So if you are taking a Multi with folic acid....stop. Find one like Jarrow's B-right that has methyl folate.
For me, taking methyl folate was like someone finding the light switch and turning it on.