Lightroom to PS stack failing

I am using lightroom and ps latest versions.  IMac Pro and latest OS.  64 GB ram. From Lightroom / edit as stack in PS.  The first one or two I do this with work fine to stack.  Each time I complete the stack focus and save the file and close it in PS. By the third image one or more of the layers fail to load and there is a white box layer at the bottom that does not clear out.  The only solution that works is to totally quit PS and launch it again.  I have tried the purge in PS but nothing seems to be there to purge (greyed out).