Problem opening infopath 2003 forms

I have just starting using microsoft infopath 2003 and am having a problem with some of my forms. I have uploaded them to a sharepoint site, some will let me open them and others I get an error message saying that infopath could not open, that the file could not be accessed. I thought I saved and uploaded them the same way but it is only a few forms that this is happening with. Can anyone help?