Hide Tanning Question

it looks like you might be letting the hide dry too much before starting to break it. to correct this, you would need to rehydrate it, re-brain, and start the breaking process over. check on the hide continually as it dries, and as soon as it begins to dry, stretch it in all directions. the drier parts should break white (like this). keep checking and stretching every few minutes. once it's dry to the touch but not quite 100%, you can start staking it (dragging the skin back and forth over a semi-sharp surface like the edge of a table/wood board or axe head) as well as stretching to help soften it further. 

it's also worth mentioning that the brain itself does not tan the hide. the process of applying brains is what’s called fat liquoring, which is basically the same thing as oiling. the fats/oils in the brain bind to and lubricate the fibers of the skin so that it can remain soft/pliable when it dries after breaking, but you still need to introduce a tanning agent. with the 'brain tanning' process, the tanning agent most commonly used is smoke. the chemicals (specifically aldehydes) in wood smoke will actually alter the structure of the skin (crosslinking the collagen fibers), turning it into leather. so basically, after softening it you will need to smoke it in order for it to be truly tanned.