Since the update of my S4 it has been working more frequently. Another thing I noticed is to make sure the unit boots up in any mode except blutooth or pandora, it seems like this allows the phone/unit to connect first, rather than trying to launch an app while connecting. Next time try powering off your car with the unit in am/fm mode, when you turn your car on allow time for the phone to connect to the unit, and then try to connect blutooth through the head unit.