Restart the router. Sometimes it will get bad with certain MAC addresses. This can be device specific.
Go into your WiFi settings on the iPad, open the problem network, and tell it to forget the network. Then add it again. This is a more complete way of reseting your connection that renewing the DHCP lease.
These may work best in combination. Forget the network, restart the router, re-add the network.
And of course, it's never a bad idea to start your troubleshooting by rebooting the iPad.
Restart and Soft Reset
iPad: Turning on/off and resetting
None of this may help. These problems are some of the hardest to track down, and many people simply give up and buy a new router. That often fixes the problem, but not always.