trouble with iPhone AirPlay streaming

I'm getting inconsistent and, at times, unusable results when I try to stream music from my iPhone to two different Airplay-compatible Pioneer Receivers. I do not know enough about WiFi to get started.

• Symptoms: Music will not always start, or sometimes cut out. Sometimes a little, sometimes a lot.
• I've ruled out the wired network: I get consistent, excellent streaming from my iMac, wired to the network, to both Pioneer receivers and an AppleTv. Perfect streaming whenever.
• I sometimes get better results when I move the iPhone closer to the nearest Airport Express. Sometimes this doesn't even make a difference. Sometimes, having it be five feet away and line of site is better than having it be ten feet away around a corner ⚠

My network:

• Central Hub, CAT6 wiring to each room, including a connection to an Airport Extreme in one room
• Secondary Hubs connected by CAT6 cables to both an Airport Express and a Pioneer Receiver with Airplay in two different rooms. The secondary hubs are connected to the central hub. In one case, I have bypassed the secondary hub to have a direct connection between the Airport Express and the Central Hub.
• The Airport Expresses are set up as "Create a wireless network" with the same network name and password, so that they effective extend the wireless network using wires between the three Airport boxes (intermediated by a hub or hubs)

Advice? I'm on iOS 7.1.2