Many of Apple's laptops are notorious for having wireless reception that, compared to other laptops, is weak. It is usually due to the metal casing. Do you have a new aluminum MacBook?
I occasionally have difficulty connecting to *some* non-Airport wireless networks on my Mac (and have had this issue with many different Macs over the past 5 years or so). ("Difficulty" meaning that there is a looong delay in the laptop being able to see the network and slower than expected connection speed.)
Also, some other information that would be useful in helping you troubleshoot:
What type of wireless network (n, g, b)? Any security on the network? OS version.