You are targeting the wrong audience? Your prices are too high? Your quality is not obvious? Your service sucks? Shipping is too slow? Shipping form is convoluted? How is that for a start?
your product is an impulse buy but priced too high shipping is too expensive merchant forms are hard to use (I have a client making a change based on that) you don't ship to their country (I've gone to buy things this year from sites I trust only to find that the company no longer ships to me) if the product is digital they've found a copy online for free or an alternative from a more appealing vendor but mostly, I'm guessing your checkout process sucks.
Just keep making sure you're getting traffic generated to the affiliate offer. It takes time, but you can continue to see this steady increase in sales over time, if you continue to generate traffic, that's the thing at this point. Keep trying to garner targeted traffic.
I would take a look at your traffic, where they're coming from, and how they're arriving to your affiliate link. If they're arriving at the order form page, it means they're interested. So if they're not buying, I question if they have the money to buy. Take a look at your traffic geographics/demographics and see if they have the money to buy.