Please check if you have the email address correct for the contact?   

Has the contact ever replied to you from this email address? 

If not, it is likely that the contacts email address is incorrect. 

Please open up the email in your Sent items box to check if in the Bcc: field or the Cc: field that you have entered has your correct CRM dropbox email address?

Please check to see that the contacts email address is not already recorded inside another contacts record or there may be duplicate contact records for this person with the email address recorded in each record.  This would mean that the email history could be split between several different contact records. 

Have all other emails you have sent out today logged OK inside your CRM account? 

Has your email account been set up correctly on your email client?   For example if you using Microsoft Outlook email is this the email client that is set up as your default email client when you click on a contact's email address inside the CRM system?  

What email address are you using to send emails out from?  Has this email address been linked to your CRM user profile? 

If you can confirm the above and if the issue is still not resolved please contact our Support Helpdesk so that we may investigate further if needed.