Two major factors can impact your goal to not spend, which includes:
Product (ASIN) errors
Target ACoS is too low
To get your goal to spend more, you’d need to turn up the target ACoS on the goal. Turning up the target ACoS effectively increases the constraint on the optimization algorithm, allowing the engine to bid more on all keywords that are currently being optimized.
Perpetua's engine rewards keywords that are winning sales and penalizes the ones that are not driving results. In other words, Perpetua rewards an increase in sales by bidding higher on a keyword until the the target ACoS is reached, and penalizes a decrease in sales by bidding lower on the keyword.
This means that if your sales are volatile, Perpetua will keep adjusting the spend based on those movements. If you want to increase the aggressiveness of the spend, you can adjust the target ACoS higher to tell the engine to keep spending even when sales are decreasing. ACoS should be set to at least historical level or higher if you want to achieve growth.
To learn more about why your goal is not spending, watch this Ad School video: