You are using Case Flags and the flag was immediately cleared after being set.

Please review the potential causes / solutions listed below.

1. If you're using Case Flags in conjunction with Email to Case Premium, ensure that your Context User for Email to Case Premium has been specified as an API User within the Case Flags Setup Wizard.

• Email to Case Premium will create both email and case comment records. Creation of case comments will result in a cleared flag unless created by a specified API User.

 
2. If you're not using Email to Case Premium or have already configured the API User per the above then check to see if you have an auto-response rule configured for your problematic cases. On the Case Flags Setup, the "Clear Flag on Outbound Email" option has a built-in delay to allow for auto-response email messages. Any email messages sent outbound within 5 seconds of case creation will not clear the flag. In organizations with substantial automation, the processing time will be longer and the difference in time between case insert and auto-response email message insert may be greater than the default 5-second buffer, causing the case flag to be erroneously cleared. 
 
Vicasso can increase the Auto-Response Email Threshold. Contact Vicasso support for enablement and considerations, support@vicasso.com.