I suppose that you've added some lines like this:
fwrite ($bf,full_tag("FIELDNAME",4,false,$user_data->fieldname));
isn't it?
and you say that you've checked that
echo full_tag("FIELDNAME",4,false,$user_data->fieldname)
return some valid info and it isn't saved to file (at a different levels depending of the course)?
Sounds very strange!!
Can you send me a copy of your backuplib.php file? And a copy of your users table (privately)? Then I'll test it....
Ciao