iPad mini 4 not going to sleep when closing the cover

Hi,

I just received my new iPad mini 4, and put it into a nice Tuff-Luv cover I used with the iPad mini 1. While the cover fits well, if a bit tightly, the new iPad, the automatic on/off when closing or opening the cover no longer works. The Auto Block option is turned on. Does anybody knows of a possible cause/solution?

Thanx!

Paolo