The Y axis of the machine has backlash that loses motion when the axis changes direction. All of the narrow sections appear after a direction change, which suggests the axis isn’t moving correctly. The X axis probably also has problems, but of a lesser nature.
Although this writeup is for Sculpfun machines, it covers nearly all of the points you must check:
A recent discussion shows the level of attention required to go from “not right” to “much better”: