Its probably a timing issue plus a bit of a coding issue. The main thing to remember is instead of delay(); calls you should keep the network updating and checking for incoming packets.
Something like:
bool ok1 = network.multicast(header1, &data_req[0], data_req.length(), level1);
uint32_t timer = millis();
while( millis() - timer < 1000 ){ //Check for incoming packets for 1 second - adjust this as required
network.update();
//handle incoming packets here
}