AMAZON Reviews keep getting deleted

so im selling a product on amazon and i giv a few of my friends a coupon code so they can get it for free and write a review(i dont actually ship them the product) and a few days after they "buy" the product i tell them to write a review but it keeps getting deleted and i cant figure out why. They all have different IP addresses, i have never logged in to my sller account on their network, i make them write "I received a discount on this item in exchange for an honest review. All opinions are my own and offer an unbiased users perspective." at the bottom of the review and still the reviews get blocked