Slacker service as a whole (not just Model S) had some major issues over the weekend. They fixed it all last night. I am thousands of mile from my Model S, but I was having difficult with slacker too. They said their engineers worked all weekend to figure out what was wrong. Basically, on regular slacker service (on my iPad and HTC One), a lot of music/albums were just flat gone out of the inventory.
I think even after that, the apps were messing up a bit, but I think it's gotten better as things naturalize with their service.