it should work ! may be any reference issue inside of your layout. please remove everything and put "Hello World" text to check that your _AdminLayout.cshtml is working or not
I've tried the solution proposed by Yidna and it works correctly, but that would mean changing the source code of nopcommerce, and when I update the version I'll lose all the changes and I'll have to modify my changes again.
Is there a way to override the admin area views without having to change nopcommerce source code?