I assume you are doing the converting of your video...is this correct? Or are you downloading your video from another site? Obviously, if some of your videos work, they were converted correctly. If you have been doing your own, are you using the same program and settings. Is it just the anime that isn't working? If you are downloading the video, you are at the mercy of the person that converted it....there are so many conversion settings that do not work with Archos products.
There are specific settings that your video .avi must conform to so they will play correctly...even down to the audio.
Let me know,