Why aren't all my conversions showing up?

I am using Page Sense to run an A/B test off ads on FB and the conversions showing up in Pagesense are way lower than what FB is showing? My trigger for conversion is the user going to the confirmation page but it might happen a few minutes/hours later - for e..g, if they pay via PayPal, they might not go to the confirmation page until they get an email later.

Is it a session issue i.e. it's only recording conversions for a particular visitor for a certain time? I need to figure this out ASAP otherwise will need to look for other solutions.

Thanks!

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