Hardware debouncing using 74LS14 - inverter input does not drop below 800 mV
TTL inputs source current when driven with a logic low. So as Solomon comments you must pull current from the input to ensure that the input goes low enough in voltage for it to be seen as a logic low. A resistor that is too high in value will not do this.
The 74LS14 typically requires a maximum of 0.4mA when held low, which would cause about 400mV across the 10kohm input resistor. Are you sure it is an LS device and not a standard 7414 that would require 0.8mA giving the 800mA that you see.
A CMOS version - 74HC14 would be a better choice of device. The input current is essentially zero for those.
As others comment a reduction in the resistor value would also sidestep the issue although the time constant would be reduced as well unless the capacitor is increased.
In general, it is not recommended to use pull-down resistors with TTL.

74LS14 Datasheet