Videos will not convert

I have no problems with my Zen Vision so far except from the fact the it will not convert videos. Some videos that do not need converting, tranfer across fine. However, when a file requires conversion, it goes to a different screen to convert the file. I use the default settings. Once it tries to convert it after literally less than 30 seconds it states that conversion was unsuccessful. It does this with every single video that needs converting. I'm not sure what I'm doing wrong and what i need to do to resolve this. Can anyone help? (The files which I tranfer are all .avi files with some .wma files. Some of the .avi and .wma files do not need converting and tranfer across okay so i don't think it's the file type that's the problem.)