My wolf whiskers black wolf synthetic works perfectly with the SV soaps, I tend to load from a fairly dry brush and add water gradually to ensure plenty soap is loaded in the the brush, it could just be down to lathering technique or maybe just not a very good synthetic, I've never had any problems lathering a hard soap or triple milled soap with a synthetic
Wife came looking thinking I was washing up after dinner, but no I was just borrowing the brush lathering, got much quicker result than with tooth brush and lather was thick and slick just like with my shaving brushes, still not happy with face feel...