missing photos in Photos

I have a lot of separate photo libraries which I manage using PowerPhotos. When Photos appeared,

I migrated the libraries one by one over to Photos. Recently I opened an old photo library in Photos, and here and there there are some photos `missing' - they are blacked out. In some cases I can click on the blacked-out space where the photo should be, and I get a white image (instead of a photo) with the possibility of editing the photo, just as if the photo were there. When I click on Edit the original photo magically appears, then I can do some minor editing thing and click Done, and voilà, the photo is restored to the library. In other cases when I click on the black space I just get an error msg saying

`Cannot Start Editing Photos cannot load adjustments for this image.'

I suspect that this missing photo problem may have something to do with the photo having been edited previously in iPhoto, and now for some reason Photos can't find it (??). I still have the original unmigrated library, but can't open it, as I no longer have iPhoto on my computer (not sure that would work anyway). I really think, though, that the missing photos are somewhere in memory. Does anyone know how to recover them?