can see all wireless networks except mine

I have 2 laptops already connected to my wireless network (integrated wifi card; no issue detecting the network). I am now trying to connect a laptop with an external wifi adapter and it detects all my neighbors networks but not mine (so the problem is not the card). I have had the laptop search for all networks again and the same list appears. I then try adding my wireless network manually including the SSID and WEP code but it still does not appear. I think there is a simple solution but have spent so much time searching that I am pulling my hair out. What am I doing wrong??

Had a similar problem once. Finally found that the router was set to transmit only "G" and not "B" format. The card we were using was a "B". Selected "both" in router configuration and "Bingo".