Hi,
Where do you have your MOSFET switch located? If it's an N-channel MOSFET you need to place it between the load and ground. (or use a proper high side driver arangement).

You also need to make sure that the voltage you're driving the gate with (5V?) is enough to turn to the MOSFET on, "normal" MOSFETS usually needs around 10V between gate and gource and may not fully turn on with 5V. There are logic level MOSFETs available that will work finw with 5V.

/Henrik.