Capsense is not recommended to have any air gap between Capactitve Sensor PCB and an Overlay material.
If the requirement demands an air gap, you may connect a spring between the capsense button and the overlay, this should enable to detect it as a button with better sensitivity.
You may also refer an application note "Using Springs as CapSense Sensors AN44999"
This application note considers the possibility of using springs as sensors. A comparison with solid conductive sensors and recommendations for the practical usage of springs are also discussed.