2 servos and Mini (they don't work)

Hi (I'm Spanish so my english may suck. I'm sorry)
The point is I'm connecting an Arduino Mini and 2 servos (pins 9 and 10, PWM in Diecimilia. I'm not sure if they are PWM in Mini...). When I connect them to Diecimilia with the same program everything works, not the same with mini. The servos move just for little periods or the just don't move.
First I powered both of them with the same board, so I thought maybe that was the problem, but I'm connecting now one of them with external power and the problem doesn't desapear.

What can be happening ? (It's my fist project with Arduino, so it can be something really stupid...). Thanks.

Sorry for not searchin before... The problem wast that I was connecting the ground of the servo just to the external power, when it have to be connected to Arduino ground too.

Sorry again

PS: I found out that in http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1232630853/4#4