I remember the first few sinew backed bows cracking across the back like this. I was eager to get the sinew on and rushed through processing the fibers as well as laying on the bundles. I ended up with gnarly, ropey, nasty sinew. There were fissures and low spots that I thought I would fill with hide glue. So, I used the stuff like plaster to fill everything in.
BAD mistake. Any job it did was merely cosmetic, a bad sinew job is a bad sinew job. The excess glue does not have the strength of the sinew and it will crack. My lessons learned: 1) process the sinew into thread like fibers 2) smaller bundles, well hydrated, and combed straight so all fibers are parallel. 3) work the glue into the fiber bundles,and then gently squeeze it out - I needed far less glue than I thought.