Let all the negative air out, I bet £20 they rise up again…..
Reset your air pressures (Positive first – Note what the pressure is when you first attach the pump, a smaller bet says it's higer than you'd expect it to be….) then try them again. If they drop down again its the seal between thr +ve & -ve chambers that's failed. It might just need some lubrication, but it's only an O ring. If it were me, I'd just open 'em up and fix it myself because it would be cheaper and less hassle then sending them back.
FFS don't ask me how to do it though, I can't be arsed to explain it!! 🙂