Andrew,
This sounds amazingly familiar .. are you writing those diskettes in a
1.44MB drive?
Even with the correct double density media I've had trouble writing
double density diskettes using a 1.44MB drive. Try finding a genuine
DOS 3.3 diskette, not a copy made in a 1.44MB drive. Or cut your
diskette images to disk using a genuine 720K drive. That'll probably work.
(I had similar problems when trying to boot a PCjr using a 720KB
diskette drive. None of the diskettes I made using a 1.44MB drive
worked, even with the correct media. Only diskettes made on a 720KB
drive were bootable.)
Mike