Sometimes smaller companies don't understand how to run a private affiliate program. They can drop the ball by not responding to emails, making it difficult for affiliates to be paid, or acting like their affiliates are competition instead of supporters of their business.
I dealt with one nightmare vendor recently and you won't believe the things that this company did. I was a huge supporter and fan of their products and had written wonderful reviews about them and even devoted a whole site to reviews for their product.
Learn my sad story in this entry about how not to run an affiliate program:
Don't Run Your Affiliate Program Like This