Recently losing buy box on brand new listings…HELP

I have my own private label and sell clothing of designs I create. I have had no issues ever in the past with the buy box but recently I will list a design on 4 different styles(Long sleeve, short sleeve, hooded sweatshirt, and crew neck) in various sizes. Within a day or 2 I will lose the buy box on just some variations and its completely random on which ones. I am the only seller and its driving me crazy because I have never had this issue and it makes even less sense that small and medium are fine but large and XL have no featured offer. Any help on this would be greatly appreciated as I feel like I have tried everything and just cannot make any headway.

Thanks!