I don't use Sound Forge, so I'll just throw this out there for those more familiar to comment on...
Could it be a hard drive space issue? Does Sound Forge require a certain amount of hard drive space in order to handle edits and however many levels of undo that it has? That's the only thing I can think of that would cause a program like that to open only part of a file -- as if it reached the limit of how much it could handle with the disk space available.