Reducing parasitic capacitance in momentary latching circuit
There is nothing discharging the 330n capacitor except your voltmeter and leakage (internal leakage should be minimal unless the capacitor is defective, as should be switch leakage). 

Perhaps your SMT board was assembled with high leakage 'no clean' flux and you are experiencing the ill effects of that, from breathing on the board etc. This may appear to be a digital circuit, but leakage resistance of a few M ohms or less could cause it to stop working. 

You could stop using that **&amp;&amp;$# (clean the boards you have with strong solvent and a brush) or you could try reducing the resistor to (say) 100K and increase the capacitor to 3.3uF.