The 10 bit settling time of your enormous 100µF capacitor will be 0.7s, so unless you are waiting nearly a second between analog reads you will simply be seeing this cap in the process of charging.
100nF is plenty, in fact 10nF is probably enough. And I second the advice about using higher values of resistance - 10k in the lower part of the divider is a good value to choose. Then you'll only drain 0.5mA from the high voltage you are measuring rather than 5mA as you are now. The existing 20k resistor will be getting quite hot you'll find.