Motor stall detector with a voltage divider in parallel to the motor and ADC: it does not work as expected
Your H bridge uses diagonal pairs to drive the motor in one direction. If the motor stalls only the coil resistance exists across the bridge and the current may rise ten times (10x). It is not a short circuit, nor is it a short to 3V or 0V because the switches are now simply low resistance current switches, that seem to be well matched.

The commutated current thru the motor inductance is 1% to 10% max of the start/stall current due to coil resistance.  The transistor switches need to be high gain to be low loss.

To pull the voltage down, you need to lower NPN impedance with a much smaller base resistance or choose NPN's with high hFE. 

To get good saturation voltage Ic/Ib=10 so the base R should be 10x the motor coil resistance.

It still won't reduce the filtered voltage to zero but it will shift down by the ratio of Rs of the switch.  Rs=Vce(sat)/Ic and varies inversely with current.