XPS 13 - 9310 - NEW - Battery - Charging ?

Hello, 

Configuration 

- Dell XPS13 - 9310 New 

- Connected with a unique USB C cable to a Dell UltraSharp 25 USB-C : U2520D (For power / display / data)

At home, the PC is always connected to the monitor. Sometimes the level of the battery shutdown to 94%. 
When I open Dell Power Manager, I see that the PC is connected but not charging.

You know why ?