Arduino Micro doesnt work with external power supply

Hi fellow Arduino geeks

I have a project going which needs an external power supply. If I plug my Arduino in micro via the mini usb, the program starts and everything is fine.
However, if I use the same powersupply and connect it with Vin and GND, the Arduino power led goes on but nothing else happens..
What could this problem be?

Kind regards

Does your sketch's setup() function have a while(!Serial){;} in it because you copied from an example that had it without knowing what that means?

DrAzzy:
Does your sketch's setup() function have a while(!Serial){;} in it because you copied from an example that had it without knowing what that means?

To be sure that it wasnt about my code and really about the supply I uploaded the 'blink led' sketch. With the mini usb connected it blinks, with the external power it does not... Even though the power led is on.