how do i get consistent results with generative extend?

I used a placeholder mp4 to extend a shot of a woman closing a door. When I replace it with the highres material (same frame size/rate) and attempt the same extension, I get totally different, unrealistic/illogical results. She opens the door again, or appears to push it off its hinges. I burned through all my credits (10 attempts) without getting close to the result I got in 2 attempts with the mp4. What am I doing wrong? And why is there no option (or i can’t find it) to top up my credits without having to subscribe to monthy extra credits I don’t need?