I get asked quite often that why an e-commerce site such as a Shopify store is not getting better sales results from their respective social media marketing ads (whether on Facebook, Pinterest, Instagram, etc.).
My first response is do you have google analytics installed properly on your Shopify store and are you using google analytics? Most installations of google analytics that I come across are not fully enabled meaning the demographics, enhanced reporting, e-commerce, remarketing, etc. are not enabled.
Assuming the analytics is installed properly and fully enabled, what needs to happen to understand why sales are not what the expectations are. Analytics will tell you exactly how much traffic came from those ads, what city those visits came from, what the average session durations and average # of page views per visit, what the bounce rate was, and what was the conversion rate was.
If you have an odd bounce rate (either very low or very high) and a low average session duration rate, that's a good indication that those visits were not real visits so the reality is your losing money on those. Another indication of spammy visits is if a large number of those visits came from just one location like a particular city (unless you targeted a particular city).
If you average session duration is say less than 10 seconds that visit was highly unlikely to have been a true potential customer anyway.
If you are simply not getting a lot of traffic from those ads the likely cause of that is the ad itself and that is a marketing issue not an issue with your Shopify store.
The point is, the accurate way to determine why your Shopify sales are not meeting expectations is to examine your analytics.