We rarely have play dates at our house for many reasons. Our home is crowded and we don't have a good play space. We are in the midst of some home projects.
We have a busy schedule between my part-time work, volunteer work, committees I'm on, hubby's schedule, and kids' schedules. When we have a weekend, we appreciate having down time with out anything extra.
With the age difference in our kids someone seems to always get left out and we just don't have the space for both kids to have someone here at the same time.
Having a playdate can be stressful for some folks even if the kids are good. I have a friend who deals with an anxiety disorder. She just doesn't have playdates at her house because it can trigger attacks.
Maybe the reason the other mom has playdates with other kids is because she is also close friends with the other mom and it is a social time for her as well.
Did something happen at her house when your son was there that has made her hesitant to have him over again? My daughter has a friend who I do like a lot. But, I don't like how she uses "potty talk" (fart, poop, butt) continually even when I ask her to stop. She has an older brother who has influenced how she talks (telling my 4 year old son that she is sexier than he is). She can also "talk down" to my kids at times ("You like THAT? I think that is dumb."). She's overall a nice little girl and not a problem to have around, but there are some things that irritate me. I don't particularly like having her at my house. But, it also isn't something that I think is a big enough deal to say something to her mom. I just really limit how often she comes to our house.
There can be a lot of reasons why someone might not reciprocate a playdate.