I had a similar issue, however, 1st mistake was, I mistyped my mac address while doing it (I suggest not actually using yours, but a spoofed one using macchanger). My second issue was that I was doing this before I did a fake auth. The fake auth attack created a working packet that I could use with packetforge to create an arp-request packet, that I could then inject succsesfully. If you want to check it out, heres what I did;
tazforum.thetazzone.com/viewtopic.php?t=6611



