Exporting DNG files in PL9.2 for merging

When I try to export 100, 50 or 10 images as DNG files for merging in Helicon two consecutive files mid export always fail. The rest all export correctly. When I try just those two files again they export fine. Running on Windows 10.
Charlie

Having said that it just exported 30 images without any faults. I’m confused; any ideas would be welcomed.
Charlie