Why does this dual monstable not function as such?
Your schematic does not show pin 16 of the 74HC4538 chip. This must be connected to the VCC rail for the chip to work.

Similarly the schematic does not show pin 8 of the 74HC4538. This must be connected to GND for the chip to work.

Also note that as the circuit is configured the top section is meant to trigger on the release of the switch. The bottom section is setup to trigger on the press of the switch.

It is possible that the top section does not operate correctly due to a long fall time on the signal to the B input. You could consider replacing the C3 capacitor with something more reasonable like a 0.1uF device. The large 22uF capacitor at this site will eventually cause pitting and destruction of the contact surfaces in the S1 push button switch.