Bedridden mom needs new circulating mattress that attaches to bed frame. Any recommendations?

Talk to the Hospice Nurse. They should be able to get another mattress or at least during their Team Meeting discuss it and the doctor can order a new one.
When I had a problem with equipment the nurse would call and order the change while she was at the house. The replacement equipment would typically come within 24 hours.
The mattress is a bit slippery it is that way for a reason. It is easier to move a person or the entire mattress up or down on the bed a bit easier.

If the mattress is slipping a lot you can put a wedge or a rolled up blanket or pillow between the end of the mattress and the foot of the bed. This will keep the mattress from slipping down.

The hospice mattress likely is designed to prevent bedsores from getting worse (or from starting in the first place). These mattresses have pulsing air in them that moves about so as to relieve pressure.

And they do what they're designed to do BUT practically everyone who's had to use one complains they are uncomfortable.

There's really no good answer here. A soft fabric mattress, or even a thick mattress topper, likely will be more comfortable BUT it can't provide as much protection against bedsores.

One partial workaround (if appropriate) might be to move your mom to a comfortable chair during the day, using a Hoyer lift. Which you may be able to get from the hospice (esp. if you present your request as a medical need and not "just" for comfort; for example, to move her to a commode if/when she has diarrhea even if she doesn't).

Another partial workaround is to put coarse sheets on the bed, which make it a little harder to slip down toward the foot. And still another is to put a sturdy reusable cloth underpad under her, as you can then pull on this to get her back up toward the top of the bed if she's slipped down toward the foot.

And, yes, you could talk to the hospice, although they may have no solutions. Or google the make/model of bed to see if, in fact, there are any similar beds (ones with bedsore protection) that are likely to be any better. If you've got one of the popular Drive brand models, you'll probably find they're as good as any.

Have you talked to the hospice provider about this problem?