If you feed 5v (e.g., usb) into Vin, it's going to drop on the way to 5v from the linear regulator.
Thanks, I've just found this out. I had a 5V power adapter going into the power jack. I need minimum 7V into the power jack or 5V directly to the 5V pin.
A small note on 5V on the 5V pin, DON'T connect the USB cable to a PC when you do. You'll backfeed 5V to the PC. Most modern PC's are smart enough to disable the USB port but it's tempting luck. Does not apply for Pro Mini and Nano as they have a diode in line with the USB power.
If you do, at some stage you will be waning to use more than a few tens of milliamps
I'm not worried about damaging Arduino's, they are dirt cheap
5V directly to the 5V pin