Hi ryadre
Thank you for your post. The required format for Item Numbers when importing is 30 characters, alphanumeric. If they are longer than 30 characters or have special characters that could be preventing the import.
Please refer to our Help article - Import and export fields for more information on that topic.
Do let me know if you need further help; and if you do need help replying with the import file would help give me more context to the issue.
If my response has answered your enquiry please click "Accept as Solution" to assist other users find this information.