People search results would not always return the user's photo profile. It doesn't happen with all the users.
Could you please advise how can I fix this?
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It depends on how you have configured the User Profile Service Application. The photos are stored in a library on the My Site Host. In a vanilla implementation, they get there when a user uploads a photo of themselves to the service by editing their User Profile.
Other options for configuration exist. For example you could import them from AD if you are using another service for end user photos, like Exchange. Exchange stores the photo and exports it to AD, then SharePoint can import the photo through an import connection from the User Profile Synchronization Service (FIM) or in SharePoint 2016 an external service like MIM.
Another alternative is to use a third party tool like Hyperfish or Hyperfish Lite that is designed to evaluate your profiles and communicate with the employee to facilitate the completion of your profiles for SharePoint and SharePoint Online.