Like a new pair of shoes, your mattress will soften after a period of time, particularly in the areas you lie (sometimes described as “dipping”). This is not a problem and is actually a sign the mattress is working with the weight of your body. There is even a British Standard on this (BS3379:2005)
If a mattress has been displayed in a shop as a “floor model” that mattress may have already been bounced on many times and may feel softer than the new mattress you will be delivered. It can take a few weeks, even months, for the mattress to fully settle down.