I recently bought a now tv stick from Argos and with the stick it came with 6 months entertainment pass. I set up my stick following the directions and have not received my pass or even a code to insert into now tv.
What i would try first is go to your My Account > Passes & Vouchers section whilst logged in on here and see if there is a message with a link to click on at the top of the screen page saying " Vouchers Waiting for You".
Should there be nothing there and the information for your entertainment pass in both the Passes & Vouchers and Bills & Payments section on your account match with no discrepancies.
Then i would go into the NowTV App on the new stick that you have just recently setup and under the My Account section of the App check the account details email address or username matches the details you used to check your voucher pass on here.