OK a mate of mine used to work out Our Price some years back and if I remember it correctly he said it worked like this:
Distis (like Cargo) persuade retail chains to take singles by doing ridiculous deals that might involve them getting say 3 free copies for every 1 copy they buy and the copy they "buy" is on a sale or return basis and so they can never be out of pocket. Thus, it matters little to them if they don't sell or even put the damn thing on display.
Basically for many years now singles have been treated simply as advertising for the album by labels. Sales of "physical" singles are pretty small (wasn't there a week a couple of years ago when the number one single sold only around 4000 copies?) and the revenue they generate is also small compared to an album (i.e. £3 vs £10).
Additionally The Wildhearts have a loyal fanbase but are hardly "the next big thing" (that's not a dig, the industry is a fickle beast) and Cargo is not exactly EMI (again not a dig but the reality is the big corporate giant's releases will get priority as they will do more business with the retailer).
It is of course still possible that someone fucked up and the delivery of the records was delayed. Who knows? Either way unless the song was getting heavy air play I doubt it would have made much difference to the final sales
