Hola, te paso unos tips del link que posteo al final y que solucionaron este mismo problema, espero te ayude:
1) fijate si broadcast SSID esta desabilitado en tu router, si es asi tenes que cambiar la configuracion de /etc/wpa_supplicant.conf
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# AP scanning/selection
# By default, wpa_supplicant requests driver to perform AP scanning and then
# uses the scan results to select a suitable AP. Another alternative is to
# allow the driver to take care of AP scanning and selection and use
# wpa_supplicant just to process EAPOL frames based on IEEE 802.11 association
# information from the driver.
# 1: wpa_supplicant initiates scanning and AP selection
# 0: driver takes care of scanning, AP selection, and IEEE 802.11 association
# parameters (e.g., WPA IE generation); this mode can also be used with
# non-WPA drivers when using IEEE 802.1X mode; do not try to associate with
# APs (i.e., external program needs to control association). This mode must
# also be used when using wired Ethernet drivers.
# 2: like 0, but associate with APs using security policy and SSID (but not
# BSSID); this can be used, e.g., with ndiswrapper and NDIS drivers to
# enable operation with hidden SSIDs and optimized roaming; in this mode,
# the network blocks in the configuration file are tried one by one until
# the driver reports successful association; each network block should have
# explicit security policy (i.e., only one option in the lists) for
# key_mgmt, pairwise, group, proto variables
ap_scan=1
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si SSID no esta disponible tenes que poner ap_scan=2.
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Si esto no funciona el problema puede venir por la configuracion de NetworkManager
http://bbs.archlinux.org/viewtopic.php?pid=237887