A few factors matter most:
Relevance. Does the artwork match what the shopper is searching for (medium, style, subject, price range)?
Budget and bid. Artists set a budget for how much they're willing to spend, and a bid for how much they're willing to pay if their work gets clicked. A higher bid can help an artwork get chosen more often, but it's not the only factor.
Performance history. Artworks that shoppers tend to click on signal to the system that there is a higher likelihood of selling this artwork, and the system adapts to optimize for what's actually working.
Pacing. The system spreads out an artist's budget across the day rather than spending it all in one burst, so a piece doesn't disappear from rotation after one big morning of traffic.
Does paying more guarantee my artwork always shows?
No. A higher bid improves your odds, but relevance and past performance still matter. An artwork that shoppers engage with can outperform a higher bid.
Why did my sponsored listing show up sometimes and not other times?
The decision is not scheduled in advance. Your artwork competes for placement on each page according to its relevance, and there are constant auctions throughout the day to determine placement and visibility, so which pieces get shown will naturally shift throughout the day based on performance or engagement, yours and others' remaining budget, and the amount you bid.