IPhone can no longer airplay through my AIrPort Express?

I have any AirPort Express creating a new network and everything with the network is fine except I'm having some problems with air playing form my iPhone. The first time I connect to the network airplay works fine. Then the next time I try to airplay it doesn't work. The airplay icon is still present on the iPhone. I can connect to the AirPort Express speaker just fine and when I play a song the iPhone shows that the music is play but no sound is coming out of the speakers even though I can airplay form my laptop just fine. Every iPhone that connects to the network can airplay once. Then when they come back they can't. If I reset the AirPort Express and create a new network all iPhone can airplay again but when they come back another time and they try to airplay they can't? What's going on here? How can I fix this?