Dark areas even with lighting

Some areas in my current base are having dark areas even though there are torches...
The areas that always have dark/no light is in my netherwart farm where I've put some nether racks and soulsand...
At first loading it has lights but when i go outside far from my base when in came back it became dark all of a sudden, it also happens when i go to sleep and check my farm...
This have happen to me several times already...