Is it normal for a topper to slide off the mattress?

I have the Simmons BeautySleep Westminster Palace DT mattress and it has a 2 inch topper which is attached to the mattress. Other than the sides, the topper is no longer attached to the mattress (which I don't think is normal). If I pull the topper up from the center it looks like a huge tent.

The other problem with the mattress is that the sides are caved in. Instead of the sides being vertical like "|", they look more angled like "/" (hope that makes sense). Because of this I can't rotate the mattress. If I have the mattress on a certain side after about 2 hours of sleep the mattress has slid about 2 feet off the box spring.

I requested the mattress be replaced under warranty (it is only 1.3 years old) and they say they won't because I didn't have a center bar in my frame, which I believe is a crock. There is nothing wrong with the box spring, it still looks brand new and I dare them tell me the topper issue has anything to do with the frame.

I need some thoughts from some experts on this and toppers in general. Is this a normal occurrence? Should the topper be attached to the mattress in some manner? Would my frame have any bearing on the sides, seeing as there is no issue with the box spring?

Thanks in advance as this is really stressing me out.