Nine in the fourth place tells a querent to check the ground rather than the effort. The reading concerns work being done properly in a place that cannot repay it, and the classic's image for that is a hunter in an empty field. Nothing about the person's diligence is in question, which is what makes the line hard to hear. The problem is location, and no quantity of additional persistence changes a location.
In practice this covers a role that has stopped containing anything, a relationship kept up in a form that no longer holds either party, or a line of work continued because it was once productive. The commentary sharpens it by adding the word long: it is not merely being in the wrong place, it is going there for a long time. In a hexagram that praises constancy, that is the one form constancy can take which produces nothing whatever.
When the line changes, the upper trigram Zhen becomes Kun and the figure becomes Pushing Upward, King Wen 46, in which wood grows out of the earth by degrees and the counsel is to advance and to seek out the person who can help. The destination names the remedy exactly. What replaces hunting an empty field is a slow accumulation somewhere that will actually feed it, and the movement recommended is upward and toward other people rather than sideways.