You haven't said which Arduino board you have.
Measure the voltage on Vin.
I bet you see +5V.
My guess is that the voltage regulator isn't working with 5v input
because the Arduino uses a very simple power regulation circuit
on the power input plug. Because of that, the input voltage must
be higher than 5v or the regulation circuit won't be able to
reliably supply 5v.
--- bill