Problem with gateway not transmitting in 915 Mhz

1 I use a RN2903 end device for US902_928 to join the loraserver.
2 gateway receives and forwards to gateway bridge.
3 The lora app server receives joinrequest, sends joinaccept
4 gateway-bridge forwards it to the gateway
5 gateway is not transmitting although the downlink frequency in set in the lora_pkt_fwd configuration file. There is not even a message in the gateway logs that the gateway received something for transmission.

the end device uses uplink channels 8 to 15 and channel 70 on the rn2903. all other channels are disabled.

Also there is no connectivity problem between gateway bridge and gateway.

What could be the reason for the gateway not showing anything and not transmitting ?