If you have sent your invoice by email, check that you received SY's notification email in your inbox. If this email tells you the invoice has been rejected, it will not show up in the portal. You must issue a new, corrected document.
If you are uploading to the portal, make sure it is supported when validating the issuance and that the uploaded invoices are present in the "Invoices being processed".
If you still cannot find one of your invoices, please post a request in the SY Information Centre specifying: the issuing account, the upload date, the name of the uploaded file, the invoice number and the recipient client.