Problem with Automation Series

Hi, My team and I encountered a problem recently. We have already sent out our campaigns via Automation (auto send out after 4 hours to those who have already opened the email for the 1st time). There are a few listings that have opened for more than 4 hours and it has not been auto triggered to send out. I have already checked the settings (auto send after 4 hours after the receiver opened the 1st send out email campaign). Please help. Thank you and regards CH