Layered navigation sometimes not working

[csdenboer]

I'm having trouble solving a pretty strange bug of the layered navigation module. I have enabled two filters; one for subcategories and one for color. The subcategories filter is functioning correctly. However, the color filter does only work sometimes and I am unable to reproduce in what cases it does work and in what cases it does not. It seems to be random.

 

You can test it yourself here: http://www.cocoono.nl/46-koffers

 

Can anyone tell me whether this is a known bug and if yes, what steps I should take to solve it?

 

Thanks, Chris