Not able to change wp_title using add_filter

wp_title filter is deprecated since WordPress 4.4 (see here). You can use document_title_parts instead

function custom_title($title_parts) {
    $title_parts['title'] = "Page Title";
    return $title_parts;
}
add_filter( 'document_title_parts', 'custom_title' );

In custom_title filter, $title_parts contains keys ‘title’, ‘page’ (the pagination, if any), ‘tagline’ (the slogan you specified, only on homepage) and ‘site’. Set “title” the way you like. WordPress will keep your configured format to then concatenate everything together.

If you want to override WordPress title formating, then use pre_get_document_title and provide to that filter a function that takes a string and returns the title you want.

function custom_title($title) {
    return "Page Title";
}
add_filter( 'pre_get_document_title', 'custom_title' );