Sorry for the bit of thread necromancy but I thought it might help someone in the future.
The issue I was having was that when you write out to a Serial connection, it queues up all of the chars. It is only if you send the int 19 that it will push the cache out.
So, if you want to send from the Arduino to the phone the string "cool" you would have to do:
bluetoothSerial.write("cool");
bluetoothSerial.write(19);
This does the trick! The 19 is also sent at the end of the string from the phone to the Arduino so if you just loop back what is received, it works.
Anyway, the magic int is 19!