I had the same problem.
For some reason I'd connected NRF_CE to Pin D4 instead of D9.
Simply putting #define MY_RF24_CE_PIN 4 made all the problems go away but I lost a couple of days figuring that out. I assumed the radio was connected fine as it was talking to the gateway, it would just not ACK messages.
The gateway would show:
7222879 !TSF:MSG:SEND,0-0-6-6,s=255,c=3,t=8,pt=1,l=1,sg=0,ft=0,st=NACK:0
(Running 2.2.0.rc2)