Hi ! Go to a dollar store they usually have sheet clips ! It is a piece of flat elastic with a clip at each end . To use these they go across each corner , under the mattress clip it across the corner to hold the sheet in place .
You can buy Sheet Clips, a bit like braces or suspenders ,for the purpose, or use velcro maybe........
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Hello. Sometimes it takes adding an accessory like fitted sheet suspenders or replacing your elastic to make it tighter.
Mattress have gotten very tall over the years and generous deep pocket sheets seem to be more common than the fitted sheets of years ago.
Alternatively another option would be to sew more elastic on your sheets to alter to make a tighter fit. Heres some helpful links that might give you more ideas and details. I hope this helps.
I simply cut 4 pieces of sturdy elastic about 10 inches long and sew across the corners. To be sure of fit you might want to place the sheet over the corner of the mattress to get the placement you want. Then just use your first one as a guide to do the other 3 corners.