I recently rooted my phone and after doing so had this problem.
I did some searching on the net and a few things were mentioned. One was going into your settings and into the wifi..when in wifi settings hit the menu button and go into advanced this will give you more settings.
A couple things mentioned were go into the WiFi Frequency Band settings and check the settings, if its set to auto change it to 2.4 or 5 ghz only...some have said this fixed the problem.
Also check the Wifi region code and make sure its set to your region, mine was set to Europe but im in the US.
NOW with all that being said it still didn't fix my problem.
I had skipped my wifi setup after rooting and resetting up the phone thinking i could just take care of it after the fact. This was NOT true, i ended up wiping the phone and reinstalling the new OS and went through the wifi setup on reboot....all has worked great.
There's a few things you can try...good luck.