I cant say your using the wrong flux as Id need to see you use it.
However your certainly not using the right flux.
The simplest is ordinary borax.
However that wont take a lot of heating. so google for Johnson ever
all round brazing flux for all your silver work.
Its highly reactive, water soluable, and dissolves copper and silver
oxides very well. Use lots of it, all over your work it will also
leach out fire stain as well.