Since the Pokki is constantly running as a background application, it's possible you can't update it because the main EXE is in use. Press CTRL + Shift + ESC to bring up the Task Manager, then switch to Processes and locate Pokki.exe in the list. Force close it, then use Software Informer to download the application again and update it as necessary. It should work this time.
Additionally, you can download the application from the official website as well.