Why is my WiFi signal very weak?

Dear all,

Recently I have changed by iPhone 3GS back cover with a new one, but I have told the service guy to to keep my old speaker/microphone/connector (all come included with the cover) and he did, but after i have the new cover fixed, I am having problems with my WiFi as it is very weak and I have to be very close to the router in order to get a signal, what could have caused this problem, the new cover is somehow blocking the WiFi signal or that tech. guy has done something wrong?

Thanks,