We're sorry to hear about your experience with your sheets.
All One Bed sheets are 100% long staple cotton, so while there is some stretch, there is not as much elasticity as with artificial fiber fabrics. With a 15' deep pocket, they are made to fit a variety of mattresses sizes, up to 15' thick. We have tested on mattresses with deep pockets to fit properly. However, with thicker mattresses, there may not be much excess fabric to tuck under.
If you could please send a photo of the fitted sheet on your mattress to firstname.lastname@example.org, along with the thickness of your mattress, we can take a closer look. We're glad you are loving the feel of your new sheets, so we can see if there is anything we can do to help you. Thank you.