Pagination not appearing on tag pages

I use the following function

function wp_pagenavi($before = '', $after = '', $prelabel = '', $nxtlabel = '', $pages_to_show = 6, $always_show = false) {
	global $request, $posts_per_page, $wpdb, $paged;
	if(empty($prelabel)) {
		$prelabel  = '‹ Previous';
	}
	if(empty($nxtlabel)) {
		$nxtlabel = 'Next ›';
	}
	$half_pages_to_show = round($pages_to_show/2);
	if (!is_single()) {
		if(!is_category()) {
			preg_match('#FROM\s(.*)\sORDER BY#siU', $request, $matches);
		} else {
			preg_match('#FROM\s(.*)\sGROUP BY#siU', $request, $matches);
		}
		$fromwhere = $matches[1];
		$numposts = $wpdb->get_var("SELECT COUNT(DISTINCT ID) FROM $fromwhere");
		$max_page = ceil($numposts /$posts_per_page);
		if(empty($paged)) {
			$paged = 1;
		}
		if($max_page > 1 || $always_show) {
			echo "$before <div class='Nav'><span>Go to page: </span>";
			if ($paged >= ($pages_to_show-1)) {
				echo '<a href="'.get_pagenum_link().'">&laquo; First</a> ... ';
			}
			previous_posts_link($prelabel);
			for($i = $paged - $half_pages_to_show; $i  <= $paged + $half_pages_to_show; $i++) {
				if ($i >= 1 && $i <= $max_page) {
					if($i == $paged) {
						echo "<strong class='on'>$i</strong>";
					} else {
						echo ' <a href="'.get_pagenum_link($i).'">'.$i.'</a> ';
					}
				}
			}
			next_posts_link($nxtlabel, $max_page);
			if (($paged+$half_pages_to_show) < ($max_page)) {
				echo ' ... <a href="'.get_pagenum_link($max_page).'">&raquo;</a>';
			}
			echo "<div class='NavEnd'></div></div> $after";
		}
	}
}

Pagination works correctly and is displayed on for search, categories etc.

BUT is not displayed for tag pages. I only have a common archive.php file so that can’t be the issue.

Help!!!