My wife and I have had an 11 inch Sealy TruFoam for almost 2 years now and we fight with the sheets every day. Its the material that the matress case is made of. It has a grain or direction. Its smooth one way and rough the other. SO the sheet slips up but pull back up when pushed down. We have tried everything. The longest we have gotten without the sheets coming off it about a week.
It sounds like you're not using a mattress pad. The purpose of a mattress pad is to protect the mattress from stains. A stained mattress voids the warranty of any mattress. besides offering that protection, it would also solve your problem.
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