If you fancy a £5 statement credit - link your Amex to a TripAdvisor account and post a review. Deal link.
Needs expiring. No longer on offer with Foursquare !
As in 'Limited to 15,000 Cards'. If you tried when it was first posted that would sound like it wouldn't be the case, was just a thought.
Card allowance ? - I tried this when the offer was first posted, and never use one of these foursquare deals before.
Possible the card allowance is used up.