You may be getting thin spots where the shaft is bending near the center. Also, there are terms for the grain configuration called rift and reed. The rift is the layered grain that appears as lines and is stiffer and harder. this is where you usually glue or attach the cock feather of your arrow. A different jig feeds the stock into a series of blades like a three bladed pencil sharpener. This is how broom handle turning lathes are designed and the three blades exert equal pressure on the shaft and there is less likelihood for variation in your diameter. Why not just buy 5/16th diameter dowelling instead of turning down a larger diameter dowel?
