I may have worked out why I have issues with my boar brush. With my badgers it's easy to fill the whole brush with lather, right down to the base of the knot. But my boar is very dense and thick at the base, and doesn't really allow lather to build there. The base is just wet. There may be lather on the outside, but inside it's just liquid, even if that water is a soapy mix.
I'm sure that watery stuff in the base is what causes the lather elsewhere to break down after a couple of passes.
I'm sure that watery stuff in the base is what causes the lather elsewhere to break down after a couple of passes.