It could be cross-compatibility issues if one type of ppt is saved on Office 2007 and others are saved with Office 2003 or older. There is an issue between documents saved on either platform if trying to open with the two different versions. That is to say there is more of a problem if using an older version of MS Office and you are attempting to open a file saved by someone who is using Office 2007. So I'm not sure why you are having difficulties opening the files.
sorry about a paragraph that doesn't really make sense reading it but I hope you understand what I'm trying to say.
