I've had EXACTLY the same issue. I thought it was due to my Google Chrome being logged in under a Google account that isn't linked to my spotify. But several days of emailing Spotify and calling Google have gotten me nowhere. I can't believe how bad sorrysp customer service is.
I've just lodged a complaint about the customer service which took another hour by chat. I'm going to highlight this issue to a few newspapers tomorrow as these free offers are being used to entice people when subscribers like us are being neglected.
Good for you - it's been really frustrating and have just been sent from one organisation to the other both claiming it's not within their power to fix or issue a new promocode. Still trying to get my subscription back from Spotify.
Get on chat and escalate it as a formal complaint. Use Twitter to and bombard them. They are lazy and careless in their customer service, they don't check the file history and keep sending the same condescending roles. It's infuriating.
You can see my Twitter complaints. I'm @jasperdeber