It is likely you are sending those emails from an address not recognized by your Pipedrive company account, and thus not emails are not allowed to arrive into Pipedrive account.
You need to send the emails either from the email address that you use to log in to Pipedrive or from an address you have added as an alternative email address.
To add an alternate email address to your Pipedrive user account, go to Tools and apps > Email sync, and add your email in the 'Alternative email address for Smart BCC' field.
When added, the email address will receive a verification email. The link in the verification email will need to be clicked — to verify that you own and operate that email address — before the email address can be used for Smart Email BCC.
To avoid having to forward emails at all, you can also switch to our Advanced, Professional, or Enterprise plan and enable Email Sync!