new theme is not being recognised

I am encountering a problem when trying to use a different theme from the two suggested versions: wordpress says all I need to do is:

You can find additional themes for your site in the WordPress theme directory. To install a theme you generally just need to upload the theme folder into your wp-content/themes directory. Once a theme is uploaded, you should see it on this page.

I uploaded it in filezilla, however, I can not see my chosen theme on wordpress.
What am I doing wrong?