Problem with connection when i close the device under my hand

Good evening everyone,

i'm starting to program with the nRF52832 development board
and i am testing the example programs. I started with the project:

- ble_app_hts_pca

The device connects successfully with the Android app when the
connection occurs in free space.
If i close the board in my hands, the connection stops working and
the app no longer receives the temperature data.

Can you help me to understand why this happens.

If I use a headset and put the antenna completely in my hands 
transmission is successful and sound is good..