Thumbnails not displaying in new Photos app included in 10.10.3 I recently upgraded our family iMac to 10.10.3 from 10.9.  I migrated the iPhoto library to a Photo library almost immediately after upgrade.  I am seeing the following behaviors occur 6 or more hours after the migration completed:


Thumbnails display as blank white rectangles for some photos in the library, including some taken last year.
Double-clicking on those thumbnails to open the photo will open the photo for viewing with some of them, for others, it will display a only the blank white background of the interface.


Do I need to be patient and allow Photo to index all the photos, thereby rebuilding the thumbnails, again?  Can I instruct Photos to regenerate thumbnails again?  I tried Command+Option and opening Photos, but that just repairs the library.  I have read that does not regenerate the thumbnails.  Any suggestions are appreciated.
In this case, the issue turned out to be caused by missing photos.  I discovered this when rebuilding the thumbnails on the unmigrated library using iPhoto.

This does not appear to be caused by any issue with iPhoto or Photos.  It may have been caused by other software or file system corruption.