Why shortcut [print screen] key does not always work?

This must be because you run your application with elevated privileges (with admin rights) (the UAC must have displayed a popup at startup).
For security reasons, Screenpresso can handle shortcuts from Windows running with elevated privileges only when it is installed in Program Files folder.

To fix this, you should install Screenpresso in Program Files folder which is not the default behavior:

Uninstall Screenpresso
Run again Screenpresso: Right click Screenpresso.exe and select Run as administrator
Click on the INSTALL on this computer button while holding the Control key or click on the sub-menu Install for all users:

Screenshot of Screenpresso install window

See Also...