Why Facebook Ads Underreport Conversions (and How to Fix It)

Why Facebook Ads Underreport Conversions (and How to Fix It)

If you’ve been running Facebook Ads lately, you may have noticed something frustrating: your ads seem to be driving sales, but Facebook Ads Manager isn’t showing the full picture. Maybe Google Analytics reports more conversions, or you see orders coming in on Shopify that don’t match up with Facebook’s numbers.

So, what’s going on? Why does Facebook underreport conversions—and more importantly, how do you fix it?

The Problem: Why Facebook Underreports Conversions

Facebook’s tracking has become less accurate over the last few years, mainly due to changes in privacy, data sharing, and browser behavior. Here are the main reasons why:

1. iOS 14 and Privacy Restrictions

When Apple introduced App Tracking Transparency (ATT) in iOS 14, users gained the option to opt out of cross-app tracking. Most did. This means Facebook can’t always connect ad clicks to purchases, especially for iOS users.

2. Third-Party Cookie Limitations

Modern browsers like Safari and Firefox block or limit third-party cookies by default. Since Facebook’s old pixel relied on cookies, conversions often get “lost” when users move between sessions or devices.

3. Ad Blockers & Privacy Extensions

Many people use ad blockers or browser extensions that block tracking scripts, including the Facebook Pixel. That means those conversions never get reported back to Facebook.

4. Cross-Device Journeys

Someone might click your ad on their phone but complete the purchase later on their laptop. Without proper tracking, Facebook struggles to connect these touchpoints.

5. Reporting Windows & Attribution Changes

Facebook shifted its attribution model (from 28-day click + 7-day view to 7-day click + 1-day view by default). That automatically reduced the number of conversions being reported, even if they still happened.

The Solution: How to Fix Underreported Conversions

Thankfully, you can improve Facebook’s conversion tracking accuracy. Here’s how:

1. Set Up Facebook Conversion API (CAPI)

Instead of relying only on the browser-based pixel, use Facebook’s Conversion API. This sends conversion data directly from your server (or eCommerce platform) to Facebook, bypassing browser limitations and ad blockers.

Shopify, WooCommerce, and other platforms have native CAPI integrations.
Tools like PixelFlow make it easy for no-code website builders (Webflow, Squarespace, Framer, etc.) to add CAPI without technical setup.

2. Use First-Party Tracking

Switch to first-party tracking where possible. This means tracking events directly on your own domain instead of relying on third-party cookies that browsers block.

3. Verify Your Domain

Make sure your business domain is verified inside Facebook Business Manager. This step is required for proper event prioritization under iOS restrictions.

4. Prioritize Events with Aggregated Event Measurement (AEM)

Facebook only allows you to track a limited set of prioritized events for iOS users. Choose the events that matter most (e.g., Purchase, Add to Cart, Lead) and rank them in order of importance.

5. Cross-Check with Other Analytics Tools

Always validate performance using another analytics source like Google Analytics or your store’s backend. This gives you a more complete picture, especially when Facebook underreports.

Why Fixing This Matters

If Facebook underreports conversions, you’re not just losing visibility—you’re making worse decisions. Underreporting makes it look like ads aren’t working, which can lead to:

Pausing campaigns that are actually profitable.
Underspending on high-ROI audiences.
Misallocating budget to the wrong channels.

By implementing CAPI and first-party tracking, you can feed Facebook more accurate data. This doesn’t just improve reporting—it actually helps Facebook’s algorithm optimize and find more of the right customers.

Final Thoughts

Facebook Ads underreport conversions because of privacy changes, browser restrictions, and attribution updates. The fix is simple: stop relying only on the pixel and upgrade your tracking stack.

Adding Facebook Conversion API + first-party tracking will give you:
✅ More accurate reporting
✅ Better ad optimization
✅ Smarter decisions with your budget

If you’re running ads and don’t want to waste money on incomplete data, making this upgrade is essential.

👉 Want to implement CAPI without coding headaches? Check out PixelFlow