Great service but images aren't coming out right

Both times I've ordered the custom stickers, they've been put together fast and they look good as far as the cut goes, but I've used 2 different images and both times they came out much darker than the image I submitted. One image basically could not be seen and you cant tell from the mock-up because it's the same image that you send in. So it looks right when you submit and confirm and then I guess the RBG to CMYK darkens the image and it comes out too dark to see. I tried to tell them to lighten a few shades if this was the case, but it was still the same. That's my only issue