It could be that too much lead has evaporated from these ^6 glazes if you fire too slowly.
I found a ceramics journal from 1904 which quantifies the percentage of the lead in a glaze, fired in a sagger, which vaporized per hour.
Journal of the Society of Chemical Industry, Volume 23 - Society of Chemical Industry (Great Britain) 1904
Without rereading it I think the volatilization of lead per hour at cone 6 was about 1/3 per hour. At higher cones it's quickly gone.
These commercial glazes are partially lead fluxed manganese glazes which form a green, blue or red glass, with a reflective layer of gold or silver on the surface. The key to success with these glazes seems to be having a very smooth bisque surface and a thick application that often runs. The Amaco Palladium has a tendency to pinhole on many clays which I've only seen in fluorine containing frits like Ferro 5301 and 3269.
I prefer the reliability of the ^05 gold glaze, which becomes especially gold when placed over a previously fired ^6 glaze to provide a smooth surface and protection against clay off-gassing.