Print is too dark

While the proof I received looks exactly like my original artwork, the stickers I received are darker. The text on the sticker was to be about a 50% black on a black background. What I received was closer to 70-80% black on a black background making it difficult to read. Now I have a 100 of them. Sticker Mule has always been my go-to for sticker, buttons, etc. but this kind of threw me.