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Depending on how old the files are, it's possible that they predate the version of the Visio file format our importer was meant to support. If that's the case, it may not be something we can fix on the Graffle end of things.

The easiest thing to do is probably send a sample file to us via the Send Feedback tool under the help menu; if we can look at one of the files, we'll have more information to work with.