I would expect a pull response from the network server to be logged.
# PULL_DATA sent: 2 (0.00% acknowledged)
# PULL_RESP(onse) datagrams received: 0 (0 bytes)
# RF packets sent to concentrator: 0 (0 bytes)
Check that you are following the packet forwarder downlink protocol correctly.
https://github.com/Lora-net/packet_forwarder/blob/master/PROTOCOL.TXT
Output from successful TX
##### 2019-01-13 04:58:21 GMT #####
### [UPSTREAM] ###
# RF packets received by concentrator: 1
# CRC_OK: 100.00%, CRC_FAIL: 0.00%, NO_CRC: 0.00%
# RF packets forwarded: 1 (23 bytes)
# PUSH_DATA datagrams sent: 1 (204 bytes)
# PUSH_DATA acknowledged: 0.00%
### [DOWNSTREAM] ###
# PULL_DATA sent: 2 (100.00% acknowledged)
# PULL_RESP(onse) datagrams received: 1 (259 bytes)
# RF packets sent to concentrator: 1 (17 bytes)
# TX errors: 0
##### END #####