I have 3 identical MBP Early 2011 with the same chipset in all. 2 of them have access to WiFi regardless of where they move to in my house, one only has WiFi in certain parts of the house.
Have run diagnostics, no results. Switched the Airport card with a working computer, the one that worked before still worked no matter where in the house it was located. The one that had the problems still has the same issues with the different card. Have tried setting a location for the WiFi, switching bands.