Ok,OK...Nobody panic,,,I found the solution myself!!
I discovered that the images that weren't generating the "normal" or intermediate picture, were exactly the same dimensions as the settings I had for intermediate pics (1024 px), hence the intermediate pic was not generated.
Cpg was also showing the original pic ('file') as intermediate, because "normal" pic was non-existent
I'm quessing that the solution would have been to change the dimensions of the intermediate picture in Configuration, and regenerate the pics. But as this happened with just a few albums, as my files are generally much larger than that, I manually duplicated the files inside the album, preceded the file name with "normal_" and uploaded them. everything worked.
Thanks again for the attention.