While playing with magento, I was stuck in related products section. I added couple of products, and tried to add related products to one of these products. I added 5 related products but on the front end only 3 was showing. So here is the places where I searched to fix the problem:
- First of all check if the related product status is “Enabled” from the admin product list.
- Next check if the visibility set to “Catalog” or “Catalog/Search” from the same list.
- If everything is ok, click “Edit” >> “Inventory”. Check if stock is grater than 0 and Stock Availability is set to “In Stock”.
- Finally click “Categories” link and see if the product is added in any category at all. Related product will not be displayed if it is not included in any category.
If still your related product is not showing up, I can’t help 
#1 by Sanket on June 18, 2011 - 2:04 pm
Please , do indexing for solving your problem and remove cache from admin. I hope it will help you.
#2 by narvijay on June 15, 2012 - 8:14 pm
Click on “Reset Filter” button on top of related product listing. It will list all products in table.
or you can clear cache + reindex data.
#3 by Martin Andersen on March 22, 2013 - 12:59 am
Just came by to solve my own problem and for some reason I found another solution while reading this.
Also check that you haven’t for some reason(while testing) already placed the missing related product in the basket. Because Magento will not show already added items as related products.