A question from a customer:
"It looks that I managed to have some keywords with 10/10 and I understand how I got there but I have one campaign that's not working properly and I don't see why I have 3/10 quality score in some despite the fact that the ads are correct and the landing pages have everything needed to have a 10/10."
Some keywords can be stubborn.
Remember that Quality Score is only a guide that tells you wether the experience you provide to searchers is better than your competitors or not. A 3/10 is an indication that your competitors are doing something better than you are.
Take a close look at your:
Click-through-rate: Is it below average? Then your competitors have better ads than you do.
Ad relevance: Is it below average? Then you have keywords in the ad group that are not closely related to your ads. Use the restructuring feature to regroup.
Landing page experience: Is it below average? Then your competitors are keeping users on their sites better than you are. Focus on improving your conversion rate. Although not directly related, it is a good measure of the experience that most searchers have with your landing page. Make sure your landing page is a direct extension of your best performing ad, they are a couple that should be evaluated together.