AirPlay not working correctly

I have 3 wireless speakers, an Apple Tv and an Airport Express wich i use to stream music in my house. The 3 speakers (Gear4 Airzone) dont work correctly. They keep dropping out and if i stream music to all the speakers at the same time the music quite often just stops. I have a Airport Extreme and as i mentiond an Airport express witch i also use for extending the network.

Already tried playing music with an inactive Airport Express, but that doesnt work either.

Anyone who knows what the solution to my problem could be?