Are you using 2.9.5 ? Did you check javascript console issue ? Could you check the source code of the page to see if adsmanager.css is loaded correctly on a "no adsmanager" page

If you are using the mod_paidsystem_feat, you should see a "adsmanager.css"

in the code
if (!defined( '_ADSMANAGER_CSS' )) {
/** ensure that functions are declared only once */
$templateDir = JPATH_ROOT . '/templates/' . $app->getTemplate();
if (is_file($templateDir.'/html/com_adsmanager/css/adsmanager.css')) {
$templateDir = JURI::base() . 'templates/' . $app->getTemplate();
$document->addStyleSheet($templateDir.'/html/com_adsmanager/css/adsmanager.css');
} else {
$document->addStyleSheet($baseurl.'components/com_adsmanager/css/adsmanager.css');
}
}

Try to change your template to default one, to see if the problem is not due to your template.

You can add
<link rel="stylesheet" href="/templates/theme1647/css/adsmanager.css" type="text/css" />

directly in your template, to force the loading of the CSS.

Doing that, is it working correctly ?

Note: it will be better to move your adsmanager.css to
$document->addStyleSheet("/templates/theme1647/html/com_adsmanager/css/adsmanager.css');

so AdsManager will load this adsmanager.css instead of default one