Assuming you have all the trunked frequencies programmed, it could just be that the SSD is very quiet at times. There's been a few times lately where I've wondered if they switched to digital talk groups - they've been so quiet. But if you give it some time, you'll hear them.
As I'm sure antfreq could tell you, they do a large majority of dispatches and communications via MDTs, so sometimes their air is very silent. Compare this to Sac PD, where there almost seems to be someone constantly on the air. There seems to be a big difference between the agencies and how they use their radios.
Do you listen to any other talk groups on the county system? Listening to Metro Fire or City Fire dispatch would give you a good indication if you're setup properly. Between Metro and City fire, there's a lot of traffic on talk groups 2608, 2640, 3120, and 3152.