Great sheets but will not stay on a mattress deeper than maybe 14"

I really, really liked these sheets but am returning them. Website info says it will fit a mattress up to 17". Mine is 16", so ...should be no problem. The fitted sheet barely gets to the bottom of the mattress and pops off repeatedly overnight. They might be okay if your mattress is not larger than maybe 14". Too bad, they are great otherwise.

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