Artificial grass installation help!!!

Hi, we had artificial grass installed over a week ago. Although the weather was freezing temperatures grass was removed from the roll, laid and pinned within the day. there are lots of ripples in the grass that move ad you walk on them. As you can imagine we are not happy!!

The fitter is saying that the ground work is completely flat and as soon as it warms up then The grass will lie flat. But I have read that it should have been flat before it was pinned. They have also done a sand infill. 

To me this doesn't seem to make any sense but I am not an expert... any ideas?

Thanks in advance.