Hello Michael,
Unfortunately, I don't have an Adafruit Capacitive Touchscreen to debug your project but I can do as much as I can to help you to solve that problem.
I was watching the function dependencies to assume a flow in the program. I found that the onClick function is called in the functions D4D_BtnOnKeyUp and D4D_BtnUnTouched which are called in the function D4D_BtnOnMessage, please be sure that the function D4D_BtnOnMessage is called when you touch the screen and that it enters to the D4D_MSG_UNTOUCHED case.
Another thing that I think you must check is that in the function D4D_ManageTouchScreenEvents the variable d4d_TouchScreen_Status be modified and then in the function D4D_HandleTouchScreen the program call the function:
// Send prepared message
D4D_SendMessageBack(&d4d_msg);
I hope this helps.
Regards,
Earl.