In the type of social setting you describe, whilst it was very nice of you to host, I personally wouldn't have the expectation of it being reciprocated. Not everyone, for a myriad of reasons would necessarily want to go to the bother of arranging a lunch. Some like to host, some don't, It doesn't really matter whether they live in big house or not, maybe they're at a stage in their life, when they just can't be bothered with any of that anymore. Life does move on for some. It's good enough that they're stepping up to the plate in being a supporting grandmother, maybe that alone wears them out.
I remember post natal meets ups, from years ago in our respective houses , half a dozen mothers and babies, it was bedlam, some of us finally admitted," lets knock this on the head", especially when some of the mothers produced baby number 2. Anyway, having said all that, I certainly wouldn't let it spoil the good times you're all having.