I have double checked and all the gallery.xml data correspond to the actual file names, as these were all previously working simpleviewer galleries before I installed svmanager.
I found that when I import an existing gallery into svmanager, all the files with upper case extensions are left out and was not written into the new gallery.xml file.
e.g. I had photos 1.JPG 2.JPG 3.jpg 4.JPG, the new gallery.xml file created would only have written the information for 3.jpg and not the rest, so obviously only 1 out of 4 picture appears.
I tried rebuilding but it didn't work, then I found the solution by simply reverting to the the old gallery.xml file the system backed up, and it worked.
Still puzzled about why it is doing that though, but since the solution is much easier than renaming every file, I consider it solved.
Thanks for all your help, I'm in love with these programs by the way.