Most electric fans used in windows or for spot cooling around the house have plastic blades which, over time, can loose their pitch and move less air. Simply remove the blade assembly and dip each blade, one at a time, into a pan or sink of nearly boiling water.
After the plastic has softened a bit, remove from water, give the blade a quick twist to recreate its original contours, then, while still maintaining pressure on the twist, hold under cold running water to set the new curve in place.
Repeat the process, copying the new shape for each fan blade. Dry thoroughly, reinstall in fan assembly and enjoy your born-again breeze box.