First, stop using bleach. Fabric content does matter, 100% cotton responds quicker than synthetics or synthetic blends. These sites explain in further detail.
Sandi, it's important to know what the fabric content is. Chlorine bleach does not work well on most microfiber sheets, but it's fine for sturdy cottons. A peroxide bleach is a good choice, like oxiclean, but you really need to let it soak. If it's body oil buildup, soak it in dawn and hot water.
I like 1/2c of 20 Mule Team Laundry Borax added to the wash.
Hi Sandi! If your sheets are really yellowed you could try one of these methods. It's a little labor intensive though. https://www.verolinens.com/how-to-get-yellowed-sheets-white-again/
If the fibre content of the sheets is predominately polyester and the sheets have already been washed and dried with heat, unfortunately stains may have already set. You could still try some of the methods suggested above but you may decide that it is not be worth the time, effort, and expense, as results still may not meet expectations.
Old yellowed sheets make good painting drop sheets though.
A cup of lemon juice in the wash.
Sandi....I would just add bleach.
After a certain point in time, they likely just won’t get truly white anymore. It’d be a time and expense saver to purchase new ones. You can repurpose the old or donate to a dog rescue.
Hi Sandi: Try this site: https://www.everydaycheapskate.com/how-to-get-yellow-stained-bed-pillows-white-again/ Good luck