Extracting vignettes, we get blank vignettes and black vignettes

We are scanning samples and extracting vignettes regularly, everything used to be fine.
But recently we are starting to get few blank or black ones when trying to extract all vignettes for sorting.
There will be around 50 to 60 of these bad vignettes out of 1000.
There are still measurement for those vignettes in the PID file, so I suppose this problem occurred during the extracting procedure.
We are using Zooprocess 5.07 (for a long time already).
Any way to fix this?

Thank you.
Ted