Check the Voltage switch on the side of your power supply. In the US you will need it set to 115V, but they come factory-set for 230V because plugging a device expecting 230VAC into 115VAC is safe, but the reverse is not. If the supply expects 230VAC and is only receiving 115VAC, it will still put out ~12VDC, but the current capability will be lower, so you are likely to see voltage drops when you try to draw current.
There is also a potentiometer on the power supply that can be used to increase the voltage. It can go up to about 14V. You don't want to turn it up all the way, but it's perfectly safe to go up to about 13V, and the performance will increase a bit.