The gaps are caused because of sagging that can take place occasionally when a lot of toner is layered onto the label at once. The printing system/labels are not designed to handle large bold areas of coverage, and sometimes the extra weight of that coverage can cause sagging which results in a small spot or line that does not get toner at all (like you are describing). The system we are using is a cost effective alternative to more expensive labeling options that would force us to begin charging extra for the labels included in a stamp set.
It is worth noting that the index labels have been designed as a reference tool, not an exact replica of the stamp images. They are offered as a free perk with the purchase of a $15, $24 or $18 stamp set (we began offering them for sale separately only because of customer requests). If the majority of customers feel as though the labels are no longer playing a valuable role in the organization of your stamps, we can definitely discontinue including them if that is what you would prefer. We try our best to create a balance between offering you beneficial details, like the index labels, and keeping costs down so you get more for your dollar.
This is in response to the complaints that the labels with the sets are smeared, streaked, or blank in places. One of my labels did not show 2 of the sentiment stamps.