Please note that the mirror coating should be non-conductive. if it is non-conductive, we think 7mm thickness is high for the sensor area that you have. Please attach some images of your board as well as sensors to understand the real situation.
What is the value of SNR you are getting for your sensors? If it is less than 5, you have to manually tune your sensors following the procedure mentioned in section 5.3 of the PSoC 4 and 6 CapSense Design guide.
You can use CapSense Tuner to measure the SNR and to tune the sensors. Please refer CapSense component datasheet to see the steps required for Tuning.
Please update the thread if you have any queries.