Looking at the output of the receiver using an oscilloscope, there many more than 5 pulses.
Probably due to the transmitting transducer oscillating even after the pulse train has finished.
For fairly close echoes, 20 - 30 cm, the peak voltage is around 4.8v and the pulse train lasts 1 ms or more.
The very first pulse might only have a peak of less than 1 volt, even less for fainter echoes.
At the moment, the reference voltage for the analog comparator is 0.5v, adjustable using a 10K pot.
The very first pulse might then be missed in some cases.
Experiments shall show how low the reference can be set without picking up noise.