I think I have found the answer to the 'non-tanning' problem. I was clearing out one of my folders in which I keep interesting articles I have read and I came across the following.

Darkroom User 1996 #2. "Cetahol... A new way to use it" Author: Maxim Muir. I quote from the article. "You will note the Sodium Sulphite level in the original stock 'A' is quite low because an excess of sulphite in the final working solution. This will nulify the tanning/staining action of the cetahol". End of quote.

He was comparing the original 'Windisch' formula. In his modified version. He used no Sodium Sulphite but replaced it with Sodium Metabisulphite. So it appears to me that the sulphite is the culprit. I hope someone else finds this useful.

NB: What a pity that Darkroom User ceased publication. I have yet to find a magazine that replaces it.