It may be the Williams Mug Soap that you used was manufactured after 2004. Williams was reformulated in 2005 and the change in performance was drastic. The lather from reformulated Williams is still slick, but it tends to dissipate rather quickly and is nowhere near as creamy, rich or as stable as the lather that you can whip up from the Williams Mug Soap that came before it.The only soap that I was never able to lather properly was Williams. There was some slickness to the lather, but no persistent foam. This may have been related to the fact that I only tried it early on in my DE shaving experience. I suspect that I could probably do a better job with Williams now. Nevertheless, I saw a video from Shave Nation, where Geo Fatboy also had issues with Williams.
The other soap that is very hard to lather is Ogallala Bay Rum soap. It’s a very hard puck, but you can eventually get it to work with a synthetic brush.
Mystic Waters is challenging until you learn that it needs to be loaded heavily.
All of these soaps work best with synthetic brushes and worst with boar brushes. The boar hair seems to absorb the lather more than badger hair. While synthetic bristles do not absorb either water or lather.
Williams Mug Soap was first sold in 1904, so at least it had one hundred years of solid performance.