Hi! New in Odoo.. my version is  8.0

I have a procurement (from a SO) and the item has a supplier, the warehouse check "Purchase to resupply this warehouse" is on. I think all is ok, but...
I try to make a PO from the procurement  ("Extra Information" Tab) clicking in "Create an Edit" and I get to create a PO but... when I return to the procurement and push "Save" (to save the changes) the PO number disappears! 

I`m not sure if it is a Bug or I´m doing something wrong.

Thanks for help and I hope than someday I could be the person thats help you... ;)