do mattresses take time to break in and get more comfortable?
i have a sealy posturepedic and have slept on it 2 nights. comfort rating is 7.5 and it doesn’t feel like that yet. does it take a little bit to break in? how long? it is an ultra plush and is brand new.

everyone’s comfort is different … you may have bought too firm a mattress for your liking …
plush doesn’t negate firm … try getting a down mattress pad …
otherwise, I am not sure what to tell you … it will likely take a year or so to "break-in" if it is a good bed … you may also get used to it … might even be better for your back once you learn how to sleep comfortably on it …
A mattress should be comfortable immediately. Maybe you got the wrong firmness. Maybe the store would exchange it for something that would be better for you. Call and ask. The worst they can do is say no.