You have to hold your pick so that your thumb's flesh will be able to touch the sting at the moment you pick it with your plectrum, it's only downstrokes. I have developped a technique where I can do that with downstrokes and upstrokes... Sounds very unusual, and it's totally useless.
Another advice is to change your hand's angle so that your pick will hit the string while your thumb touches it. You might want to ask a teacher, live, too... A lesson can be as effective as 1000 words.