I have a similar problem with NetSpot 1.3.335. On my Macbook Pro (2007 model 3,1 running 10.6.8) the scans are fast (about 3 sec) but only find networks on channel 1 (my neighbours’). It does not find my own much stronger signal on channel 6 (nor when I changed it to channel 11 but yes when I put it on channel 1).
On my MacMini (2010 model 4,1 running 10.6.8) scans take much longer ( about 30 sec) than on the Macbook Pro but all channels are found.
It therefore seems that on the macbook Pro scans are terminated after channel 1 is searched. I note from that the Macbook Pro has a different Airport extreme card ( Card Type: AirPort Extreme (0x168C, 0x87); Firmware Version: Atheros 5416: 2.1.14.6 than the Mini (Card Type: AirPort Extreme (0x14E4, 0x93); Firmware Version: Broadcom BCM43xx 1.0 (5.10.131.42.4 ))
Might this be significant?