You need to reset the buffer position to the beginning. You can do this by doing buff.seek(0).
Every time you read or write to the buffer, the position is advanced by one. Say you start with an empty buffer.
The buffer value is "", the buffer pos is 0.
You do buff.write("hello"). Obviously the buffer value is now hello. The buffer position, however, is now 5. When you call read(), there is nothing past position 5 to read! So it returns an empty string.