I went into my game and noticed the same. Only one sim went and got a foot massage. Most that were there went to do yoga when the class spawned up and the rest just stood around once in a while doing sit ups and push ups or what not. But the massage therapists and reflexologists just stood around reading their books as OP said. It really makes the Spa seem dead. I am also using the Maxis made Perfect Balance Spa and I did not renovate this lot either so everything is as it was originally created.
As far as the yoga class it worked fine for me. I had this problem as OP did a while ago but it seems to be fixed for me now. The problem I'm seeing are sims going to get massages they don't seem to interact with that nor do the go meditate. I forgot to add it seems as if sims arent using the steam room as well like they used to. I may get one other sim in there with my active sim. Very disappointing as my spa used to be very active in the past.
