3.0.6 mongoimport error

When I use mongoimport and mongoexport in version 2.4.12 they work fine but now that updated to 3.0.6 after using mongoexport and not changing anything on the .json file and using mongoimport not all of the documents get imported. It is not consistent to which ones get imported sometimes only one doesn't import sometimes 3 don't import. It gives me the same error for all the documents "The _id field cannot be changed.." but I have not made any changes to the document.