Filtering grouped product

Hello,

I m trying the ajax filter plugin and need a solution for this scenario:

I have a grouped product(NC 3.30), call 'Armani perfume test' with two associated products:
'Armani perfume test 25 ml' and 'Armani perfume test 50 ml' .
I would like to see a filter based on size of the perfume bottles:

25 ml
50 ml

But i cannot get this working. i tried with specification attributes and product attributes, but no success. Please some help with this.

I solved it by checking the 'enable specification filter' in the setting. works great !!!