Pellets guinea pigs not eating new pellets

My two guinea pigs Rosalina and Zelda have been on some pretty unhealthy pellets because I thought that there weren't any healthy pellets where I live, but i recently found Oxbow Cavy Cuisine and bought it for them. I've been introducing it to them slowly and been mixing it with their other pellets ( 1/4 of that to 3/4 their other pellets) and they haven't been eating the Oxbow ones. I understand that they'll obviously eat the unhealthy ones first, like offering a kid a cheeseburger and an apple, 9 times out of 10 they'll choose the hamburger, but they just won't eat all the Oxbow ones, they eat some, but they never finish them, they eat about half of the Oxbow after they've eaten all their other ones, but they barely finish them, and I have been doing this for about a week. Apart from the pellets they get a healthy diet in all other aspects, they get about 2 cups of vegges a day, unlimited Oaten hay (there's no Timothy hay where i live and i get it fresh and green from a horse supplier and they love it) fruit about once a week, and obviously fresh water in a bottle at all times. I'd just like to know how to get them to eat the Oxbow pellets because I really want them to have the best possible life and the old pellets are quite bad.