Incoming mail to Zoho CRM; only some emails are synced

Incoming mail to Zoho CRM; only some emails are synced

Hi, we upgraded to Zoho CRM professional recently. The most important thing for us is to be able to have all customer emails synced between our Gmail account and our Zoho CRM. Sending emails works fine. Email is linked to each contact in Zoho CRM and we can send the email without issues from Zoho CRM. 

Receiving emails works partially. The problem is, we don't get all incoming emails into Zoho CRM. This means that some customer incoming emails are threaded under each contact card but not all. 

Anyone has an idea why only some emails ends up in the Zoho CRM mailbox but not all. It's kind of useless for us if we only receive some emails into Zoho CRM.

Thanks!