Habilitar modo monitor:

# airmon-ng  start wlan0

Interface	Chipset		Driver

wlan0		Intel 4965/5xxx	iwlagn - [phy0]
			(monitor mode enabled on mon0)

Se tiene que habilitar el monitor mode

Ahora miramos si inyecta:

# aireplay-ng -B -9 mon0

17:32:52  Trying broadcast probe requests...
17:32:52  Injection is working!
17:32:54  Found 5 APs

17:32:54  Trying directed probe requests...
17:32:54  00:1A:2B:01:84:F6 - channel: 3 - 'WLAN_33'
17:32:58  Ping (min/avg/max): 3.954ms/19.388ms/37.586ms Power: -71.29
17:32:58   7/30:  23%

17:32:58  00:02:CF:98:97:0B - channel: 3 - 'WLAN_D4'
17:33:00  Ping (min/avg/max): 1.519ms/35.425ms/55.902ms Power: -70.66
17:33:00  29/30:  96%

17:33:00  00:1A:2B:69:95:0E - channel: 3 - 'WLAN_96'
17:33:01  Ping (min/avg/max): 12.296ms/47.004ms/82.029ms Power: -63.57
17:33:01  30/30: 100%

17:33:01  00:18:39:2A:E0:86 - channel: 4 - 'Cortes Asociados Wireless'
17:33:07  Ping (min/avg/max): 10.871ms/10.871ms/10.871ms Power: -70.00
17:33:07   1/30:   3%

17:33:07  00:22:2D:02:7C:F2 - channel: 4 - 'WLAN27C481'
17:33:10  Ping (min/avg/max): 69.518ms/83.123ms/152.368ms Power: -68.76
17:33:10  29/30:  96%