Missing plugins in admin panel: Not using subfolder, read directions for plugins

I have installed plugins before and never had an issue, but I am not having an issue installing plugins to a site for work (test.americaninstitute.edu). I have tried the following:
– installing the plugins via the admin page … what happens is that I cannot activate the plugin. It says “no header,” though when I manually check the php file, I do see a header.
– installing the plugin via the admin page and then going back to the plugins page to activate — it does not appear (though i do see the file in the plugins folder when I check the FTP directory)
– deleting those files and downloading the plugins, extracting the files on my desktop, checking to see that they aren’t in a nested subfolder, and uploading the plugin folders to wp-content/plugins.
– finding another plugins folder … the only other one is in the main site (www.americaninstitute.edu) but taking plugins out of the subdomain into the main site’s wp-contents/plugin folder did not help.
– reading various readme files to make sure I’m following the plugin creators’ directions. The ones I checked did not require anything out of the ordinary, but still … no shows on the plugins!

This seems to happen with every single plug in i try, from a very small plugin for popups to the Google Analytics plugin. I’ve searched through the forums for the answer, and am coming up dry. Please advise! Thanks!