No Chime for Second text message

My wife took my 4S when I got the 5. She installed IOS 6 as a new phone in iTunes and restored from a backup. Ever since this, she gets a chime for a first text message, she replies, then puts the phone to sleep. When she gets a follow-up text from the same person, the phone does not chime. This is pretty inconvenient and annoying. Any thoughts?