Hi and welcome to the forum! It would be interesting to know how long you've had your dog and when he was trained, although not necessary to understand that he's likely experiencing a fear period in his life. I'm putting a link to a website that explains it all fairly well, but you should also do a search for fear periods in dogs on your own.
http://www.positivedogs.com/articles/early_doesnt_mean.htmlI have a rather well trained Rottie who just turned 2, and although the last fear period was less intense for him than the others, I certainly noticed it.
They typically won't continue training narcotics detection with a dog who is not mentally sound, which is why I'm supposing this is simply a fear period, but I could also be completely wrong, knowing nothing about his previous 20 months of life.