Are you using direct link in Editor?
When you import a model / scene through datasmith directlink you should at least get a new folder with: a datasmith scene, a folder “geometries” and a folder “materials”.
Is that the case for you? Did you save those before closing Unreal?
Then you might also have an Unreal Level that contains that datasmith scene, but that is less important.
I did:
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Load Unreal, and look for the Datasmith Scene asset that was created when importing the file from Revit (Archicad in your case) => because Archicad was not running I get information that the source is not available.
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Start Revit (Archicad) and open the same 3D model/scene that the one you use for direct link before. Now the datasmith scene asset should tell you that it is not synced anymore
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Sync the model in Revit (Archicad) using the synchronise button in datasmith tab
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Right click on datasmith scene in UE and select “Reimport”
You can also do those in that order 2 3 1 or 2 1 3