Understanding op-amp Schmitt trigger circuit
I think your problem is actually that your ultrasonic sensor doesn't have a "hard" voltage output. It appears to have a high output resistance of ~10K. That means that you've got a voltage divider between R4 and the ultrasonic output. R3 is pretty much too small to care about. The ultrasonic sensor is getting overwhelmed with voltage from the op-amp. I would remove R4 to begin trouble-shooting. This would remove feedback so you won't get hysteresis, but you'd no longer have current back-feeding your sensor.

Two potential solutions:
 1. Increase the feedback resistors by 10X or 100X to lower the relative output resistance of the sensor.
 2. Buffer the sensor output with an op-amp.