Mine Support doesn't prevent cave-in

Greetings.
Today a part of my mine collapsed, even if I had built Mine Supports few days ago.

On the screenshot you can see the radius of a Mine Support (next to the mine support built) and cave-in is clearly in the radius.
(Sorry for my english)
Today a part of my mine collapsed, even if I had built Mine Supports few days ago.

On the screenshot you can see the radius of a Mine Support (next to the mine support built) and cave-in is clearly in the radius.
(Sorry for my english)