Need to press reset button on Arduino + Wifi Sheild when using external power

Hi I'm creating a project for school which will allow me to turn on an led through Wi-Fi. My problem is that I noticed that when i power my Arduino + WiFi shield using USB it works fine, but when I use an external power source I need to press the reset button on the Wi-fi Shield in order for it to work. I am going to be using an external power source and I need to to work even without pressing the reset button. Any ideas?

I was thinking that maybe I can search for a code that would reset the arduino the way that the reset button does. But all I found was a software reset which will not fix my problem.

What does that mean? What does the shield if you don't press the reset button? Is it just when you change from USB to the external power supply?

You can reset the shield by the Ar duino but that needs an SMB resistor to be soldered on the WiFi shield (bottom side, upper left corner between the two free pads).

something similar from the past.

http://marco.guardigli.it/2010/11/arduino-wiznet-ethernet-shield-proper.html