Dry lather is a common problem. The cause is that the water that used to be in the lather has evaporated, leaving you with a cake-y mess on your face. Once the water is removed from lather, all we’re left with is soap. Soap which wants to just blow off in flakes or cling stubbornly to our face.
This is a very bad problem for straight razor shaving. Even with a well honed razor, you cannot shave with dry or drying lather. The straight razor needs a slick lather; we like tallow soap for this reason. Thankfully the solution is simple. Just dip your brush in hot water (you could use cold too if you really want) and relather the dry areas. A good practice in the beginning is to shave half your face, or one third, before relathering the next area. This way, your lather will be nice and slick; making it easier for your well honed straight razor to cut through those pesky hairs.