Product filter problem

I have created a “Manufacturer” product filter which is included in a block on our site along with a price selection filter.

There are a total of 146 products, spread over 5 manufacturers.

As you can see from the image, grabbed from the homepage, it correctly parses all the items in the prices category (they add up to 146, to save you checking) but only sees items by one manufacturer. It will only show the other manufacturers when you drill down to the exact subcategory in which they reside.

It has worked correctly on the homepage on 2 or 3 occasions, but seems to be intermittent. Any ideas ?