I have been on about 7 ADs in my life. ALL had worked on my anxiety until this point and all were ssri/snri as well. But the tricyclic antidepressants she tried were worthless to me as was the Prozac (an ssri). I have no idea why the Prozac or the TCA didn't work. I have done a lot of reading on AD failures and it seems to have no pattern. Many researchers belief it depends on your genetics. There are services that do genetic testing to determine which genotypes a person has for serotonin, adrenaline, etc. They seem to have only slightly better results than good old trial and error. I think that with time and more testing that accuracy rate will improve however.
I am afraid however that there are too many variables to consider. One of my last posts was a section on what vitamins/minerals and diseases can cause anxiety/depression. There were about 20 or so vitamins/mineral deficiencies/toxicities that could play a part and over 300 diseases. So how complicated is it to pick the right AD to help? Almost impossible. That is why they call it PRACTICING medicine. lol