thumb fails to be generated

My folder structure is like this
siteroot/uploads
siteroot/uploads/images
siteroot/uploads/_thumbs

I have some images in the images folder, The first time I open up ckfinder it generates the subfolder for im in _thumbs

_thumbs/Images

But I don't see any thumbs gets generated . My app is on freebsd php 5.2.5. What's possibly wrong?

Here is the thumb config :

$baseUrl = '/uploads/';

$config['Thumbnails'] = Array(
'url' => $baseUrl . '_thumbs',
'directory' => $baseDir . '_thumbs',
'enabled' => true,
'directAccess' => false,
'maxWidth' => 100,
'maxHeight' => 100,
'bmpSupported' => false,
'quality' => 80);

Thanks for your help.