In my experiences with my teams, there could be a couple of reasons.

1. The email address you are emailing is not the same as the one on the contact.
2. The contact isn’t in the Person object in Gainsight.
3. The contact is tied to more than one account.
4. The email address you are sending from isn’t the same as the one that is tied to you in Gainsight.

Are you getting any kind of message back from Gainsight as to why it was unable to attach?

Thank you Heather,
I will try and investigate with my team.
I get the following reply: “Your email titled "###" was posted to Gainsight Timeline as a draft since no contacts were available in recipients and Gainsight could not resolve the email to a specific customer.”

Again, thank you for your reply!

I have done the following check:

The email address I am emailing to is the same as the one on the contact

 I cannot seem to find the Person object in Gainsight. 

 The contact is not tied to more than one account

The email address that I am sending from is the same as the one that is tied to me in Gainsight

The issue seems to be due to the Person object that is not available in my Gainsight. We only have Contacts.

All the SFDC contacts must be load to MDA using Load to Company Person Action (Presently its changed to “load to people”)

@lerkatron Yes, as said by @venkatesh Load to Company Person Action is changed to “load to people”. 

Refer to this article for more information and let us know if you need any help. 

Thank you all for this help!