DIAGNOSIS: The speed selector switch contacts are dirty or the switch itself has failed internally. High speed often works because it's a direct circuit.
FIX: Unplug the blender. The switch is not user-serviceable without disassembling the base housing, which voids warranty and is complex. You can try a temporary fix: rapidly toggle the speed selector switch from Off through all speeds about 20-30 times. This can scrape corrosion off the contacts. Plug back in and test.
IF THAT DIDN'T WORK: If toggling doesn't restore lower speeds, the multi-speed switch is defective. The entire control panel/switch assembly needs replacement. For the K150, this requires professional repair or base unit replacement.
HEADS UP: Liquid ingress from overfilling or wiping down the base with a wet cloth can cause switch corrosion. Always clean the base with a damp cloth, not a wet one, and avoid letting liquid drip into the button area.