Hi cowtweets,
As you’ve only had the mattress for a few days, there will be a break in and adjustment period for any new mattress or sleeping system as the mattress loses any of its “false firmness” and the cover stretches and loosens a little and your body gets used to a sleeping surface that is different from what it is used to (see post #3 here ). This would typically be at least a few weeks but it can be shorter or longer depending on the specifics of the person and the mattress.
Feeling “firmer” would be odd and perhaps more of an issue of the final expansion of the product, you adjusting to the new product and/or perhaps the additional of the mattress pad/protector and fitted sheet. Rolling around on the mattress and gently walking upon it (be careful!) certainly can help expedite this “break-in” process.
Hopefully in the next month the comfort will be closer to what you desire. If not, as least you had the foresight to choose something that offers a good exchange/return policy.
Keep us updated!
Phoenix