Incontinence bed pads (both washable and disposable) bunching up under my mom-what do?

I use both washable and disposable bed pads under my mom. We use the washable one for mattress protection but also to serve as a turning or pull up 'sheet' as invariably she needs to be pulled up a couple of inches for meals (she is bed bound and head of bed is elevated for eating). When I see my mom's back, it often has red marks from where the bedpads bunched up and her laying on them. Now I try very hard to keep the bedpads as taut and flat as possible but would love to hear what others do and other possible remedies. Thank you in advance for your help.