I guess, there is no guarantee that the upper commission will increase sales…

It is supposed to give a much higher exposure and this way a bigger CHANCE to have more sales.

Bonanza [URL removed] “Higher advertising levels unlock access to additional shopping channels, and include a higher ad spend to drive more buyer traffic to your listings.”

Check also this, it is [URL removed]

“Even if you select a high max fee, you’ll need to take steps to ensure your listings are accepted and posted to the various shopping channels we support.
Since each paid click increases your fee, make sure that your items are optimized to sell fast":

How to maximize traffic and sales using Bonanza feed partners
[URL removed]

Regardless commission %, You should still try to drive more traffic on Your own, e.g. by using social media.

If I may suggest, try to make some changes to some listings and to see later (e.g. in a period of a month), if it make any difference in Your stats.

Titles – add in the beginning e.g. Gold Bracelet or Gold Engagement Ring and then other specifications.

Descriptions – since the most important in search results are the first 160 characters of the description, maybe move this black banner down the page and don’t use this sentence right in the beginning “Please Mail us your ring size, when you purchase it.”
Add first searchable (with important main keywords) description – start again with the “14 ct Gold Engagement Ring” and then add more data and customization info.

I hope, somebody else will be able to answer more precisely, why You are “getting no sales at lower commission points”.
I think, Bonanza doesn’t run advertising by dividing items to each level up to the 30% – all items are at 30%, but I know that they try to charge after the sale as low fee as possible, it depends, though, how much they’ve already spent on advertising Your item, until it is sold.