I have an Arduino Leonardo and an Arduino WIFI Shield. They work well now.
I inserted WIFI Shield in an Arduino card, and the same software, mutatis mutandum, does'nt works:
he sends the data correctly only the first time,
he does'nt receive the site answer,
the WIFIScanWireless sample halts on Serial.println of the available networks.
When an Arduino halts (freezes), that is normally an indication it has run out of SRAM. The difference between the Leonardo and the Uno is minimal. The Leonardo has 2.5K, and the Uno has 2K.
You might try using the F() function with your static strings if you are not already.