I'm not sure what brand we have, but we have the opposite problem. Our top sheet fits nicely, with good overhang on each side. It's our bottom sheet that won't fit properly. It's just not big enough to go all the way down and fit around the bottom of the mattress like it should. We've tried two different brands and have had the same problem with both.
Maybe we need to buy a set of whatever you have and then mix and match to get everything to fit, lol.

We are also having trouble finding a comforter that is big enough to fit the mattress. We need it to be 120 inches wide, and they are pretty much non-existent. We've found a few quilts that fit though.

The maker of these big fluffy mattresses need to come up with a line of bedding that will actually fit them.

Sounds so familiar, my husband and I used to play tug-of-war all night with the top sheets, till I got a brilliant idea. I unstitched the matching pillowcases and sewed them up each side of the king size sheet. Now they go to the floor on both sides...problem solved.

What kind of King bed do you have? The standard King is the same length as a queen, but wider. The California King is longer than a queen, but not as wide as the standard King. If you have the sheets for the other type of King bed, that may be why they aren't fitting correctly.

I'd just sew a strip of muslin along the sides or bottom of the sheet to make it wider or longer. The muslin would get tucked in, so it won't show.

The Company Store sells some oversized sheets and comforters.

Here is a link that might be useful: Company Store

Have you tried Cal. king?

It's a standard king!

robin2007- I bought a down comforter from Pacific Coast which is 120" wide. Finding a 120" wide duvet is another matter.

bella, great idea with the pillow cases!

Ralph Lauren comforters are all oversized and fit nicely. I don't know about the sheets.

Thanks sujafir, that might work.

I just measured the width of the mattress and it's 109" from the bottom of the mattress over the top and to the bottom on the opposite side. You'd need a few of extra inches to be able to tuck in a sheet. I've been checking on the internet and none of the web sites give the dimensions of the fitted sheets.

Costco's kirkland signature sheets are actually 108x106 -- I have NOT seen any sheets at costco that measure 111x111

oh my yes! Super annoying! its like you know we need deeper sheets and made them as such but why have you not made them longer!!! Here is this from Amazon I have to rotate it for cal king so basically the top is the side now. https://www.amazon.com/gp/product/B01BBK975G/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&th=1 If you have the money there are also custom size sheets out there.

I’m late to this party, but 3 comments: 1. Thicker mattresses take up room that make fitted sheet hard to fit, and flat sheet not long enough or wide enough. 2. But deep pocket fitted sheets. Anywhere from 17-22.” Measure depth. Buy “oversized” flat sheet. Most are 120x112” or at least 118x110.” Just Google oversized sheet. 3. If you want to make your bed in traditional method with enough length on flat sheet to fold over top of blanket or cover, and enough width to make a hospital corner, modern sheets will not be large enough, especially if you have a thicker mattress.