I soak my mug and brush in warm water while I'm prepping. Then
I take the brush and hold it over the sink until barely any water
is dripping out of it. I then take it and creat the lather over the
soap. I get a great lather doing this. I also add glycerin, but
that shouldn't be necessary to get a good lather. As Glen mentioned,
perhaps it's your water quality that is causing issues.

John