A Reddit thread is asking folks to share their greatest SEO success story and there are some good ones. But Google's John Mueller chimed in and said his was as simple as instructing someone to remove the noindex from their site. He said "sometimes good things come in small checkboxes."
John said "Told someone to remove the setting that added the noindex to all pages. They were pretty happy; sometimes good things come in small checkboxes."
The funny thing is - I had a similar situation and I often keep going back to it when I talk to people. A fortune 500 company hired me to talk to their 10 person SEO team about an issue they had with one of their large sites not ranking well in Google. I have this policy of only doing SEO consulting for SEOs - but I generally do not do any SEO consulting - but in some cases I'll let SEOs hire me for an hour to shoot questions at me. I don't look at the questions beforehand and I don't look at the site in question beforehand.
So when the call started, I fired up a screen share, they told me the domain name and within 60 seconds I spotted the site had a noindex. As you can imagine, the SEO team was super shocked and I guess a bit embarrassed since the head of marketing was on the call as well.
But yea - sometimes it is the most basic things that have the biggest impact.
Forum discussion at Reddit.
