Decking Shrinking

[zooming]

I had decking constructed and the boards were laid but together and over time they haze recently started to part due to shrinkage (I presume), and started to free. A couple of questions, what the best way to refix, tie down and will they expand again. I have recently applied ronseal oil.
Thanks again

 

[breezer]

screw them down, not nail them

 

[zooming]

I will srew them down, just wondering whether to screw them at the egde of each board at an angle or just in the centre.