Problem with decking warping after 6 months

Hey All,
We had some yellow balau decking put down before christmas, I noticed as the weather has gotten warmer that some of the long boards are warping. I got back the landscaper who put it down and he said some of the screws had snapped. He replaced the problem ones. But then I went to check around some drain pipes area and 3 out of 4 screws just broke away. I got him to come back and he replaced almost all the screws, probably a third had snapped.
But i`m still seeing some boards coming up, and even ones that were rescrewed down arnt that flat.
See photos attached. Any ideas? Thanks

 

Would suggest the wrong screws used , should not snap .