Ya it's common problem for the potters. Usually if the atmosphere not reduction the cone will be seen clear enough. I have simple way to solve that problem, just blow it. So you just closer the peephole and blow it for several times. But remember don't too close, your mouth will burnt. I will guarantee it will be succeed.
I usually put the small test piece glazed beside the cone to make sure glaze melted well. The piece have a small hole so you can draw it by iron. You will see directly the glazes are melted well.
Okay good luck!
- Stuart likes this