Having problems getting an iPad to join or rejoin an Wi-Fi network? Apple has posted a KnowledgeBase article (http://support.apple.com/kb/TS3304) on the matter.
Apple says that, under certain conditions, iPad may not automatically rejoin a known Wi-Fi network after restart or waking from sleep. This can occur with some third-party Wi-Fi routers that are dual-band capable when using the same network name for each network or using different security settings for each network. If you encounter this issue, try the following:
° Create separate Wi-Fi network names to identify each band. This can be done by appending one or more characters to the current network name.
° Ensure that both networks use the same security type (WEP, WPA, WPA2, and so on)
° If the issue persists, reset your network settings using Settings > General > Reset > Reset Network Settings.
° Always ensure that your Wi-Fi router firmware is up to date.