What objective and event are you optimizing for in Facebook? You can get ridiculous number of initiated checkout events and low purchases, if you optimize for conversions and that event.
Is the cart abandonment data you're seeing matching up with these high abandonment rates in the Facebook pixel?
Seeing that you will have captured their email at that stage, doing an abandoned cart email series plus retargeting these people through FB will help push them over the line. On top of that, (and through that), you need to hear from people's mouths as to why they haven't ordered. Get out of your head or here, and go to the source of the problem to find out.