Plush mattress turn out to be firm

I bought a slumberland pocket PLUS PLUSH bed about 6 weeks ago. I do understand that a new bed will usually be firmer when new. Now 6 weeks later, no difference. It is still firm. The pillow top has a softer feel but after a while laying down its get hard. Now i have neck and shoulder pain daily. I bought this mattress specifically as I needed the opposite of firm