If most of your long tail keywords just give you traffic with no sales then there could be any or all of the following problems

Irrelevant landing page (No relevancy)
Weak product description
No extensive information product features
No Call to actions
No Company Logo
No Sign of trust
No proper content presentation format
Poor product image
Weak Navigation
Sloppy website design
No proper placement of shopping cart
The product price might be higher
Long checkout page and other factors.

take care