volume keeps going to zero

I have an S5 with an official Samsung case and the volume keeps going to zero so i miss calls and texts. At first i though that i was catching the volume keys as i took the phone out of my pocket. I tried altering the volume by pressing the volume keys through the case but it did not work. In fact when you shut the flap of the case it closes the screen of the phone so the volume keys do not work.
I have checked the setting and i can't see anything that would cause this.

Anyone have any ideas??