Two reasons this happens, and neither is something you did wrong.
Only shops that have joined print the code. If your regular shop is not on ZlipR yet, tell them about it — most are happy to join, since it costs them nothing and their billing does not change.
When a shop has no live offers, its billing system is told not to print the code, so you do not end up scanning something that could never pay.
Without a code on the bill there is no way to credit that purchase, so it is worth checking the app for live offers near you before you shop.
Still stuck? Reply to any ZlipR notification or write to us from the app.