RC Schmitt Trigger Debouncing - Cap discharged but there is a voltage across it still
You have chosen the SN7414. If you look at the schematic in the data sheet page 3, you will see that internally there is a 6K ohm resistor tied high to Vcc and then through a diode to the input pin A of the 7414. So your capacitor is being charged up by the input from the 7414.

Either use the 74AHCT14 (or equivelant) or reduce your resistors to values low enough that your resistors can soak up the (almost) 0.8 mA (edit, I originally said 8mA incorrectly) from the 7414. Meaning your R2 needs to be a lot smaller.