Most blower motors are power by a basic electrical circuit that utilizes a multi resistor or a solid-state resistor to control different fan speed modes (low, medium low, medium and high). If the resistor malfunctions it may lose one or more of the lower fan speed settings.
This resistor is almost always located in the heater plenum so it can be cooled by the air circulating through the vents.
It's a fairly easy job for a DIYer.