I'm not sure why it's not working.
Did you try with the defaults but only changing the "page" and clearing the before/after?
wp_link_pages( array(
'before' => '',
'after' => '',
'link_before' => '',
'link_after' => '',
'next_or_number' => 'next',
'separator' => ' ',
'nextpagelink' => 'Next page',
'previouspagelink' => 'Previous page',
'pagelink' => '%',
'echo' => 1
)
);
Alternatively you could always "hard code" the links to the single.php.
I mean that doesn't really fix you issue, but you know..
<?php global $page, $numpages, $multipage, $more;
if ( $multipage ) {
if ( $more ) {
$prev = $page - 1;
if ( $prev ) {
$link = _wp_link_page($prev).'Previous page</a>';
$output .= apply_filters( 'wp_link_pages_link', $link, $prev );
}
$next = $page + 1;
if ( $next <= $numpages ) {
if ( $prev ) {
$output .= ' ';
}
$link = _wp_link_page($next).'Next page</a>';
$output .= apply_filters( 'wp_link_pages_link', $link, $next );
}
}
echo apply_filters('wp_link_pages', $output);
} ?>
Modified straight from post-template.php/wp_link_pages()