K.B.
Not all people react the same way around young children. Some are naturals at acting like a kid themselves and getting in there and having fun and others just aren't. If they are doing something that you do not like (disciplining, etc.) then you need to address that with them so that it ends, but if it's just a matter of wishing they would do more I think you need to relax and just let the relationship form the way it's going to. How were they as parents? Were they affectionate and playful or a little more reserved?
I remember that my paternal grandparents were the "fun ones" and my maternal grandparents were not. As an adult, I loved them both, but as a young child I enjoyed going to my father's parents' house much more because they kept toys in the house for us and would actually play and have fun.
One thing I would consider is the personality of the two sets of grandparents. My MIL is very outgoing and never waits for an invitation to do anything. She would call anytime, anyday and say I'm in the area I'm coming over. My mother on the other hand will wait for an invitation. If we left it up to her to invite herself over we'd never see her. It's not that she doesn't love her grandkids, it's just that she doesn't want to be a nuisance and wants to be invited.
I would just be careful about comparing the two grandparents and just provide them opportunities to bond with your children in their own way.
Good Luck,
K.