DIAGNOSIS This points to a problem with the heating element or its connection. The element is at the bottom of the tub, so if it's not working, the bottom rack gets no direct heat while the top gets residual warmth from the fan.
FIX First, run a diagnostic to see if the heater is getting power. Run any cycle and listen carefully during the final drying phase. You should hear a faint humming/buzzing from the bottom—that's the heater on. If you don't hear it, the element, its terminals, or the high-limit thermostat might be bad. Visually inspect the heating element (the looped metal rod at the bottom) for any obvious breaks or burns.
IF THAT DIDN'T WORK The issue might be the "Sensor Dry" system itself. A faulty humidity sensor can tell the machine the dishes are dry prematurely, cutting the cycle short. This often requires a service call to diagnose and replace the sensor.
HEADS UP Always use rinse aid. Without it, water droplets don't sheet off properly, and the sensor can get confused, leading to inconsistent drying even if the heater is fine.