Have you signed up for and been accepted to their affiliate program? (if no, well that explains it right there)
If you are an Amazon affiliate, the next question to ask yourself is are the products you are offering relevant and useful to the Hub topics you write about? Are you trying to sell items?
If you are just writing about "whatever" there may not be any reason for any of your Hub visitors to go shopping. You might find the Hub I wrote about Hub earnings to be helpful,