Servo only starts talking to your sims if he runs out of other things to do. Remember, he acts as a maid, a gardener and a repairman in that order. As I said in my article on this site (see here
), he is a passive host, not an active host. Very rare that Servo will start talking to somebody while there's something outstanding.
So if you have a party, say, and people keep disappearing off to the bathroom(s), Servo will usually go in after they come out to clean the toilet at some point. If the hot tub then breaks that will go on Servo's "to do" list. If the genie then kills all your plants, Servo will usually (but not always) put replanting on his list of things to do. And so on. If nothing else happens, Servo may then decide to start socialising.