I have a few sites that are indexed in Google (when I do the "site:url" operator), but they do not show up for their main keyword in like the first 20 pages.
I know that this happens to new sites, but a few of these sites are like 6+ months old and had been in the #3 position all this time for their main keyword. Then they disappeared for a few weeks and then came back for a few days again, but now gone again.
I've experienced this with new sites, but usually after building some backlinks they become stable and keep their position.
So what is the main reason while my sites which have been around for a while have disappeared from the SERPs while still being indexed? What can I do to get them back in the SERPs?
Any feedback would be helpful.