Problem. Robust colour transition

[89design]

Hi guys. Working on A1 posters with various car designs. Yesterday received first 4 samples from printing house and not very happy with result.

Instead of smooth colour transition it looks very robust and thicken. Before going crazy, maybe somebody could guide me, what might be reasons for this? Is it even possible to expect better result?

Design - https://www.dropbox.com/s/5e1rtjmd1k3ki ... m.jpg?dl=0

* Happens f all 4 posters
*Happens aprox. in the same places

Looking forward for advice