canoscan 9900f does not scan proper area

[JayElDee]

Hi, new to the forum and relatively new to scanning. I am trying to scan 35mm and 126 and 2.25 slides and negatives. I have a Canoscan 9900f and Canon toolbox 4.1.3 on my mac os x.

I have the scanner working and I am having trouble getting the scanner to know what is the area I want scanned. It will chop off much of a slide as in the example
http://i74.photobucket.com/albums/i2.../Scan20001.jpg

I had four 35mm mounted slides laying on the glass in the center. They were all mangled this way. I have tried different areas of the flatbed and pushing the edges of the slides together to no avail. Photos appear to scan normally

what am I doing wrong?
any help appreciated

John