Post Thumbnails not Appeared All

Hello,

I have almost 1600+ posts in my blog but the image thumbnail image does not shown off all the related posts. I have selected 5 but some time 3 and some time 2 thumbnails show. Other shown default image.

So how to solve it that it should show all the image thumbnails.

Thanks,

Asif

http://wordpress.org/extend/plugins/related-posts-thumbnails/