Wish I could say for sure what the issue is for you. I've seen the same thing happen when I try to interact with certain sims, but in pretty much every case where that happens the sim in question is in the process of leaving the lot, akin to the "this sim is going to work/home" situation in TS3. I am able to interact with other nearby sims at the same time, so I know it's specifically that sim who is non-interactive.