No one seems to have a solution to the flat sheet on adjustable base issue…
I have an adjustable base, and use a suspender system to keep the fitted sheet on the mattress. It works great.
BUT, I like to use a flat sheet too, and no matter what I do, the foot of the flat sheet always comes unfolded during the night. Every morning, I have to tuck it back in under the mattress. I really wish someone would make a flat sheet with elastic just at the foot of the bed, but I can’t find a way to solve this problem.
Any Ideas? Suspenders don’t really work because the flat sheet still comes untucked, except where the suspender is clipped on.
After reading the description of your problem I have what may be a solution for you but it would require a bit of sewing, probably just a few minutes time if you have access to a sewing machine. As long as your flat sheet is long enough you could fold over the end and sew along the two sides to form a pocket of approximately the thickness of your mattress. Then just slide that pocketed end over the bottom end of your mattress and tuck in the sides and I think the bottom should then stay in place. Might want to try it with an inexpensive sheet first to be sure it would work to your satisfaction before modifying a costly sheet. Hope this helps!