A question mark (?) in your Dock indicates that the application has been deleted. If found, Snapz Pro X 2.5 or later will delete previous versions the application during the installation. This will result in the question mark if you have added Snapz to your Dock previously.
Please note that Snapz Pro X was designed to be used via its hot key. This ensures that you can use it for just about any capture. The installer will not place Snapz Pro X in the Dock. Since Snapz is a background process, it will not be displayed as a running application in your Dock. While you are free to add Snapz Pro X to your Dock, we do not recommend it as it can lead to confusion and potential problems.