Decent thickness but very grainy print

I've ordered these before and been really happy with the image quality, but this time it came out quite grainy - my source art wasn't and was sufficiently high resolution with no issues pointed out in the proof, so I'm not sure what happened. I did get them on deal so perhaps the QC was lower because of that? Either way they're decent enough for the price but I'd be cautious as the print quality seems to be hit or miss.