some books can only open to the beginning of the current section (file).
Some books only have 1 huge section
Next time you find a problem book: (remember to CANCEL when done looking)
Unpack it (U key for the selected book): Explode (button):
Look in OEBPS folder: look in the TEXT folder: those are the sections.
Only a few? One really big?
Splitting (use the Calibre editor) may help