Theme not showing up in Manage Themes

I have uploaded a new theme via ftp into
wp-content/themes

But when I go to the manage themes section it is not showing up.
I can see twenty twelve and another theme that I uploaded a while ago. But this theme is not listed in the wp-content/themes folder. In there are 6 other themes I have used over the years but there is no sign of the one theme I can see in my manage themes page.

All a bit strange.
Any ideas where the manage themes page is pulling it’s files from if it isn’t wp-content/themes. Strange when it has obviously pulled files from there in the past.

Any suggestions?
I am using WordPress 3.6.1

Thanks