there are coded uniqueness defined in VFileImport.java (Import File Loader) for Table i_product. The columns UPC and Value must be unique in the Datafile. Also the catenated BPartner_Value+VendorProductNo must be unique. It is not a database constraint. The columns are nullable. No message is displayed if the rules are violated, but the records are discarded.
see http://www.adempiere.com/Data_Import#Import_File_Loader_discards_input_records
to fix the bug I modify ImpFormat.java but I cannot push the changeset of BF3564464. I am not a committer
to reproduce the problem I attach UPC-field to ExampleProduct.csv
can be reproduced with idempiere: Add UPC-field-definition to ImportFormat "Example Product" and load attached csv-file.
I solved the problem under
https://bitbucket.org/eugenh/idempiere/changeset/be01240201d96ae502fef663d11215c3513722a9
So any ADempiere-committer can review it and merge.