Help with creaky/moving flooring in modern house

[Blackcurrant]

I'm looking for a builder/surveyor who can advise on firming up a creaky and slightly squashy area of flooring in a modern house. There is too much give in the floor, as though the boards (chipboard) are not settled on the joists or the joists have moved. If anyone has rectified a similar problem or can recommend a builder to sort it out, I would be grateful if you could let me know. thanks

[Blackcurrant]

Thanks. I haven't lifted the boards yet but it's a good idea. I suspect joists need attention as the problem affects a large area.