Emails in the same conversation are not grouped together

Emails in the same conversation are not grouped together

Hello!
I come from Gmail, and I notice that the grouping of emails into the same conversations are only sometimes working. On many cases, they are not grouped at all, even though they are clearly in the same conversation. How does Zoho Email group them, on what keys/tags etc? I think you need to look into it, as it is quite annoying. 

This is just one of many examples: