A few things here...
1, You are not doing a good job pre-framing them before they buy. You need to set them up about what they will get with the product and how it can help them
2. The vendor may have a shitty sales page that does not do a good job explaining the product (which is why you need to do this for them)
3. The product just may suck. Ask for a free copy (they will give you one seeing you made 10 sales already) and read over it. Would this be something you would find value in if you had heartburn issues?
If the answer is no, them move on and try to find another product in the heartburn niche that offers affiliates to advertise for them
If the answer is yes, then the product is good and you need to go back to step 1 and do a better job of pre framing the product (pre framing is basically getting them set up for the sale...letting them know they are about to buy something without actually selling them on the product)