For some reason my Discreet Rendezvous isn't working any longer. I enter the spa then immediately leave. Any help?

Are you selecting the Visit Lot command or a Rendezvous specific command to get the sim inside? The former only sends the sim to a rabbithole lot, it doesn't tell them what to do once they get inside. I used to make this mistake with Grocery Stores and Diners occasionally then wonder why my sims are just standing outside.

Another possibility is that maybe you don't have any other Rendezvous members or service sims available at the time with the required Level 1 or higher KamaSimtra skill to interact with?

If those are not it, perhaps try MC Resetting the Spa lot or replacing the Spa?

I click on the spa, then click on "Discreet Rendezvous". The sim walks in, the door closes behind him, and a few seconds later he just walks out. Unless something got changed, Discreet Rendezvous doesn't require townspeople to be listed. I used to use DS right at the start of the game. All the available sims were listed at YSH (Young Adult, Service, Homeless) ranging in price from 200 (level 1 KS skill) to 2000 (level 10 KS skill ... you can't go wrong marrying one of them, haha). All of them had jobs such as mail carrier, maid, babysitter, pizza delivery, etc. The problem persists across all towns, so the spa itself isn't the problem. It also persists regardless of how developed a particular game is. I also forgot to mention that Matchmaker also no longer works. It does give me a list of sims, but all of them are 0000 regardless of how many other sims I have met.

Have to admit that although I play with KS professionals, I haven't used Club R lately at all. Will check on this the next time I am in-game. But otherwise, I must defer to our developer/resident Club R specialist for more feedback as to why this might be happening.

Are you currently playing in Isla Paradiso or are you playing in a world where the Spa even though it's on land is below the sea level as I've had that happen in IP and when the Spa was technically below sea level.

Like I said, I have this problems across all worlds. Currently I'm in Twinbrook. As for the sea level thing, I can't say that I've ever seen the spa below sea level.