Our CO2 pressure changes appropriately and keg is about half full but no beer comes out of the tap?

Inside the sankey tap, there is a little backflow protector. It is usually a little plastic ball. In a bar, the taps get poured pretty much every day, and these don't tend to be a problem (plus, most distributors send a 'tap cleaning guy' for free montly or bi-monthly). In a home kegerator, the ball can become stuck, blocking beer flow out, instead of in. It is easy to clean. Some people just remove these. I do not recommend that as it can open your keg to infection, or beer can get into your CO2 line if you run out of gas. Inside the sankey tap, there is a little backflow protector. It is usually a little plastic ball. In a bar, the taps get poured pretty much every day, and these don't tend to be a problem (plus, most distributors send a 'tap cleaning guy' for free montly or bi-monthly). In a home kegerator, the ball can become stuck, blocking beer flow out, instead of in. It is easy to clean. Some people just remove these. I do not recommend that as it can open your keg to infection, or beer can get into your CO2 line if you run out of gas.