Related products are not shown in some products

Hello Sir,
As you know related products are shown on every product page.
It is shown on the basis of category and tags.
But in some pages it is blank and some pages "No related products" are written.

So why it is showing blank,atleast it show "No related products".
Please check these urls
http://aureusart.com/avani/store/products/aureusart-6/
http://aureusart.com/mridula/store/products/untitled-vi/

Please let me know..
Thanks