I've tried a couple more times, adding the water gradually, and there is a very narrow sweet spot between paste and soup in which I get a rather thin lather. Takes a lot of work, though.
I'd thought of trying distilled water, but frankly, I'm not sure I like this stuff well enough to put in any more effort. There are other soaps that lather much more easily for me (Sapone di Paolo, Wet Shaving Products, Italian Barber, to name a few).