555 one shot missing a beat
You mustn't be switching VCC!
By doing that you're relying on the power-up behaviour of the ICM7555 every time. And that's undefined, far as I read the datasheet. So, what happens isn't that surprising: you're taking away the power supply, and that puts TR, CV, DIS and THR all above the supply voltage. That is outside of what is allowed for operation. In fact, the datasheet even says that's outside of the maximum (damage-free) regions. So, that's a design fault!
Instead, keep VCC at a constant voltage (e.g. 5V, or whatever between 4.7 and 18 V youhave at hand), and only change the voltage at the TR input.
(also, unless you're doing anything else with the same pulse, too: The job of a pulse generator is to make defined-width pulses. Not quite sure you really want your circuit if you have a pulse generator. Also, resistors and large capacitors are not exact at all and thermally stable; a (7)55 is really not an acceptable approach if you need precision in timing.)