Imports are so fiddley, it's tough to know what to suggest. What fields are you trying to import? Have you turned off the option on the import template to "add records"? Can you change just a couple of records without a problem? Are you imprting from Excel or CSV? What fields are in the import file? Does it make a difference if you limit the file to just item code / picking sequence?

You can send me you Inventory files and the file you are trying to import, but I won't be able to look at them until sometime tomorrow when I'm in the office. You're consultant will be able to help in a shorter time frame.