Capacitive Sensor gets stuck
The problem is that you're touching the foil.

The "capacitive sensing" used in that library is crude at best.

If you read the Arduino Playground page for the Capacitive Sensing Library it discusses a lot of the problems with the library, including grounding.  Make sure that the Arduino is properly grounded, or provide your own ground plane under the foil sensors.

Also they talk about the value of resistor and the sensitivity.  With a 10MΩ resistor you get sensing at (supposedly) 4-6cm from the foil.  That's the value you're seeing as you remove your finger.  While you're touching the foil it's just having a complete brain haemorrhage and can't work out what is going on (completely unable to charge the pin since you are leeching off all the current).  For "absolute touch" it recommends no more than 1MΩ.

Of course, as I mentioned, that method is rather crude.  To do it properly you need to:


Discharge an analog input pin through a MOSFET to ground (low impedance).
Apply a constant current (typically 50µA is used but varies depending on ADC characteristics) to the analog input for a fixed amount of time.
Remove the current and measure the voltage on the ADC input.
Repeat.


More modern MCUs have this kind of functionality built in, in the form of a "CTMU" - Charge Time Measurement Unit.  If you want to have anything more than "there's someone vaguely in the vicinity of the sensor" then I would suggest switching to a more modern MCU and doing it properly, or employing a specific CTMU chip as an external device.

Also, the sensor should be comprised of three parts - a ground plane, a sensor plane (connected to the ADC) and a dielectric layer between them.  The sensor should also be insulated from direct touch.