Layered Navigation Not Listing All Products

[cricket-hockey]

I've just realised that the layered navigation block in 1.6.1.2 is not filtering all sizes.

 

For example, if I have 20 products in stock in size large, when I tick to filter by large, only 6 products are appearing.

 

It seems to be doing this for all sizes - only including about 1/3 of my products.

 

I think it's just the more recent products that I've added to my shop which it doesn't filter.

 

I've tried recreating the search/urls/prices/clearing cache etc but nothing works.

 

Here's an example of a product which has 2 items in stock in size Harrow.

 

https://www.cricket-hockey.com/gm-mana-f45-606-junior-cricket-bat-2016

 

If you go back to the category page and click Harrow in the filter, the product doesn't appear

 

Any ideas?