Something about the vertices is preventing automatic weights from being computed. Likely if you move the vertices slightly it will resolve the problem. The reason this can happen is due to floating point precision. When a vertex is very close to an edge, it can sometimes be considered on the wrong side of the edge. This can cause Spine to think edges are crossing when it is computing weights, and that causes it to fail.
If it happens every time, you can post or email the project along with instructions on how to see it happen. We'd be happy to take a look! contact@esotericsoftware.com
