Facebook Conversion API - Events are tracked in events manager but they are not showing in ad manager. What might be the reason?

Question

Its been 15 days days we are trying to figure out this mystery. We have integrated Facebook conversion API in one of our projects. Below is the code for reference. All the events which we fire using this are tracked in events manager. However those events are not tracked under ads manager. Ads manager count always shows 50-60% lesser count compared to events manager. We also understood that ads manager will show only attributed events. However what we are not able to understand is why is meta considering nearly 50% of our events which have fired from conversion API as non attributed then. What might be the problem?

We have received success response from meta for all api calls

 {"data":"{"events_received":1,"messages":[],"fbtrace_id":"AQGISAFu--xmHSxebPKS69m"}","http_response":200}