Can’t activate plugins

I’m new to WordPress… I click on the “activate” link for a plugin… I don’t get any errors, and the page displays a message that a plugin was activated… BUT I don’t think it really is active, and the link does *not* change to “deactivate”. I don’t think it’s active because a theme I’m using that needs the plugin gives errors about not finding a function named like the plugin… Running on a Windows server if it makes any difference…