I have a 2012 Honda CRV (w/o Nav) and the iPhone 4s. I was having problems with Pandora not always working correctly. Finally, I think I have found the proper sequence of steps to follow. This seems to work all the time for me now if you do them in this order:
1) Turn on the power button for the radio
2) Change the source to BTA (blue tooth audio)
3) Start Pandora on your iPhone (do not start it before this step and do not plug into the USB port yet)
4) Wait for the BTA to pick-up the audio (it sounds better and louder than the USB connection)
5) After audio is playing via blue tooth, plug in the phone to the USB port
6) Press the AUX button to change the radio source from BTA to USB
7) Enjoy Pandora
I put a copy of these instructions in the Notes application on my iPhone in case I need to refer to them.