The 802.11a/c standard specifies backward compatibility with 802.11a/b/g/n. However, it wasn't until the fourth generation Airport Extreme came out in March of 2009 that they were based on the 802.11n standard. Prior to that time Apple was forced to use the 802.11n draft standard. The devil is in the details, especially in network protocols, and there were some compatibility issues between devices at that time. While I have not encountered it personally there could be a compatibility issue between draft 802.11n devices and 802.11a/c.
If your Airport Extreme is dual band (2.4 and 5 GHz) capable, be sure the 5GHz band is enabled. It might make a difference.
