March sales invoices in gstr1 in the month of may

Hi Everyone, 

I am not able to upload March Sales Invoice details in GSTR1 in the month of May. We being Exporters, by the time we received the Shipping bill copy, the due date for April was over. In the Invoice, March date is mentioned and in Shipping Bill-April date is mentioned. 

But, as April`s GSTR1 date was over, I am trying to upload that Invoice in May but the protal is not accepting. Did anyone face the same problem.

Regards