DIAGNOSIS The most common culprit is the mode selector. You've likely got it set to a low-torque mode (like precision driving) instead of the high-torque impact mode.
FIX Check the collar behind the chuck. Turn it so the "hammer" icon is aligned with the arrow or line on the tool's body. This is the standard impact mode. The numbers (1-3) are for lower torque/speed settings.
IF THAT DIDN'T WORK The brushes in the motor could be severely worn, reducing power. This is a wear item, but failure this early is rare. Also, try a different battery—a weak cell pack can show full charge but deliver low power under load.
HEADS UP Don't confuse this with the auto-stop mode (where the tool stops when the screw is seated). That's a different function. Weakness is about rotational force, not automatic shutoff.