When I downloaded and installed the All-in-One SEO plug-in for WordPress and it didn’t work quite right. The new <meta> tags were being displayed, but the <title> had not changed from the WordPress default setting. The reason was the tag <php wp-head()> was in the wrong place.
The All-in-One SEO plug-in interacts with the page through the <wp-head> tag, if it is missing; the plug-in cannot work at all. My plug-in was working, just not fully. That led me to test the placement of the <php wp-head()> tag. Sure enough, it was above the <title> tag in the <head> section of the page.
So, if you want the All-in-One SEO plug-in to work on your site, make sure that the <php wp-head()> tag comes after the <title> tag in the <head> section.