Adding logos warps video

Hi
I'm new to Davinci Resolve. I'm having an issue adding a logo to a video. I can add the logo, size and position it but when I do it changes the video. Sometimes the video becomes still with audio playing over it. In other cases, it repeats one frame over and over very quickly. The logo is not always there in the frame.

Undo doesn't resolve the issue and the only way I can find to fix it is to delete the project, start again and leave the logo out, which is not really an option.

Can someone tell me what I am doing wrong and how to fix it?

Thanks