I followed @Frank Kern ’s directions to the letter to sell a book of mine, investing $5/day for each type of audience. It worked great for a few weeks and I sold several hundred products, but then ... suddenly ... everything stopped. I re-did the same campaigns, changed copy, image, audience, etc. But nothing. Now they are slow, consume little budget, and above all, bring no more sales. Zero. Any idea what it could be? Thank you for any suggestions.