DIAGNOSIS The drill is likely in a high-torque, low-speed gear (setting 1 on the collar) or the clutch is set to a low number. It's bogging down because it's geared for power, not speed.
FIX Check the adjustable clutch collar behind the chuck. Turn it to the drill icon for maximum power without clutch slip. Also, ensure the gear selector (above the trigger) is set to "2" for higher speed when drilling into anything but the hardest materials.
IF THAT DIDN'T WORK You might be using a dull bit or drilling without a pilot hole in hard material. The motor lacks the raw torque of higher-voltage tools and will bog if the bit isn't sharp.
HEADS UP Forcing it like this repeatedly will overheat and kill the motor. Let the tool do the work. If it bogs, ease up, use a sharper bit, or pre-drill.