Why are printers frustrating?

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So as a graphic designer, and someone who is into customized items, i have to deal with printers a lot. A few weeks back i made a customized iphone case for a client but the problem was that the print out was of poor quality, the colors of the pictures where downgraded but I didn't say anything. I simply begged and apologize to the client about the poor print out, and even promised to give him some discounts on his next jobs. Fast-forward to the 30th of June, the same client contacted me that he wants to do another phone pouch and begged that this quality should be better. I contacted the printer and explained to them how they did a poor job the last time, and even went further to show them proof with the picture i sent them and the print out. They apologized and promised to do a better job this time around, only for them to print this out and it was worse than the previous job. I got angry for the first time and told them I can't accept such job, they kept on telling me it's the printer but there where other phone pouches they printed there that had decent quality, so i asked them if it was a different printer they used for those jobs they said so, so why is mine this bad? I told them i can't accept it but they where unwilling to do another print. The time was already after 7 in the evening i left the shop angry without taking the job. If the shop refuses to do another one, the only option i have is to either do a refund or find somewhere else, and both options is a huge loss for me, these printers are just frustrating my business.