CP4 has stopped converting my CP3 files...why?

Hi all.  I can't find a similar question posted in obvious related posts; so... I have CP4, and used to be able to open an older project created in CP3. You could watch it  convert the slides then provide option to save in v4 as new file name.  Now I cannot do this, beyone the Save as diag box option, then ...nothing..  I had a reinstall of programme - which has not resolved the issue; and - yes, I am working locally, and only small number of frames.? Lucky I still have CPv3 but am frustrated by this.  Is this by design?  Thanks for any help or advice.