We have just had some turf laid ( a week ago today). It is v soft and spongy, we have water it copiously as instructed, my husband is v worried it will never settle and harden up a bit as we had it levelled at the bottom end and lifted about 6", I know it's not been v long but any tips would be welcomed, he's getting paranoid! We had it done professionally, it has been laid on sand and top soil.
just give it time. It needs to root into the soil below before it will begin to firm up and the new soil needs to settle down which will take a while. Also you need to water it well as you are doing but this will always make the turf feel spongy as it is full of water.