My ipod wont join to a network but it can join to others
You can try resetting the phone and the network settings and see if that works. If it doesn't, connect the iPhone to iTunes and update the software, or restore the phone. Check out more tips on www.pcworld.com/article/210480/iphone_annoyances.html.
try resetting the router for the network it won't join. Unplug the router for 30 seconds and then plug it back in and let it reboot.
if its on ios 6 or 6.0.1 than that might be why. Those firmwares hav problems connecting to wifi