there may be numerous reasons.
MAC Filtering is on
there may be some diff. technically according to rev. number in both the AP's.thus sometime aireplay fails against some of AP's
go a little far from AP & check that same freq. spectrum & interference not being created by Microwave owen
try with iwconfig rausb0/mon0 rate 1M
check you are getting SSID in airodump or it may show length:1 or length:7 according to hidden SSID length