Edinburgh hits can, like relationships, go stale. Shows that were fresh and exciting in August away from home can, months or years later, settle down and lose their spark. They get a bit of money behind them, pile on some stuff and, suddenly, they're not the shows they once were.

Dirty Great Love Story was a big fringe hit in 2012: a playful little rom-com that toyed with the form's tropes. Written and performed by Richard Marsh and Katie Bonna, it laid out their love story in full – from a drunken one-night stand in a Travelodge suite to a full-blown public declaration of love – and all of it written in rhythm and rhyme.

True to the form, it takes two years to get together: plenty of com, not that much rom. After the sort of post-stag shag that neither party really remembers, the two of them keep bumping back into one another – at barbecues and Christmas dos, weddings and christenings and shoulder-crying sessions. You know, the merry-go-round of your mid-thirties. They're more off and on than Martin Creed's lightbulbs.

Five years on, Pia Furtado's peppy production lands in the West End and it's still charming, still fun and still laced with good laughs, but it's lost the thing that made it click – namely Marsh and Bonna themselves. Here, they're played by Felix Scott and Ayesha Antoine – both totally likeable, but both ever so slightly miscast. Scott's too classically handsome for his geeky counterpart; Antoine, too together for her muddled-up singleton.

Back then, the game wasn't ‘will-they-won't-they?' (It's a rom-com, people, I promise you they will.) It was ‘did-they-or-didn't-they?'

This wasn't just a dirty great love story, in other words. This was their dirty great love story and there they were, up onstage, in public, sharing it with strangers, reliving it daily, owning it entirely. Did she really puke in his lap mid-blow-job driving down the M6? Was he really screwed by her breakfast-thieving ex? Each one of its improbable rom-com clichés stood as proof that love can be every bit as ludicrous as it is in the movies. Love stories, it promised us, really do happen. Don't they? "This is real life, people," Marsh insists. "It's not a story."

Only this time, it is, no doubt about it, and without that doubt, all those anecdotes flatten out. There's not the same shame in making them public, nor the same flicker of feeling at play beneath. Clubbable and energetic as Scott and Antonie are, their presence makes the whole thing feel processed, even hackneyed. One way or the other, it rings false and the moment it does, Dirty Great Love Story becomes bog standard: a rom-com like any other.

Because there's nothing beneath the surface here: no deeper ideas about the tightrope of love, no interrogation of the pressure to pair up. Just two characters missing the moment and misreading the signs again and again, taking an age to get it together. It's Richard Curtis all over again – only this time it rhymes. And Richard Curtis in rhyme is the stuff of cheap Valentine's Day cards. Harrumph.

Dirty Great Love Story runs at the Arts Theatre until 18 March.