It may not be anything personal against your or your child. I'm very busy working full time and all and have limited time to host playdates. Just yesterday I finally had the chance to reciprocate a playdate that my son had about 2 months ago. It had been on my mind but I just never was able to schedule it in. And yes like this person, I've had other kids over before I had time to recipricote for various reasons. One was the other child I invited over had my son over about 4 months ago! So it could be she's so behind in reciprocating that it's not your child's turn yet. It could also be that she has other kids over because she's babysitting. My son has had another friend over more often than some of his classmates who he is much closer with because the mom & I watch each others kids every now and then. So it looks on the outside, like he's having a playdate but in fact I'm sitting, although they are playing.
It could also be that the parent is embarassed to have your child over her place if she thinks her place is not up to par with yours. Or her place doesn't offer as many choices as yours.
If your child would like to have another playdate with the child just invite the child over again. Yes it would be nice if she hosted as well, but don't let that stop the kids from having playtime outside of school. It's about the kids and sometimes we have to remove our emotions for the better of the children. There could be a myriad of reasons but its not worth contemplating when the kids could be having a good time together.
However, if you feel strongly about her hosting, just come out and say "hey, my son would love to come over your place and play with _______. Does next week Tuesday or Thursday work for you?" And see what she says.