Forever never seems that long until you're grown, and after 21 years the Wu-Tang's claim to immortality is under heavy questioning. It's been a full seven years since their last group outing, 8 Diagrams, but far from being refreshed the clan has returned tired, grumpy and overweight. A Better Tomorrow is billed as a 20th anniversary celebration, but it arrives a year after the fact, without any hit singles and with the collective barely able to disguise their total disinterest. The RZA has dug deep to will this thing into existence – reportedly at great personal cost – but just out of shot of the record's blue-skied sleeve is a raincloud twice the size of Staten Island.  

This same storm has been brewing ominously over the clan for at least half of its lifespan, but the disputes that plagued 8 Diagrams have only intensified with time. Earlier this year Raekwon declared himself "on strike" from Wu-Tang, while Ghostface has continued his tradition of releasing solo albums within a week of the group effort. GZA, meanwhile, spends his time rapping about the big bang to university students, now more interested in chemical than hip hop elements. Method Man is still getting stoned with Redman but stopped releasing records when the money dried up, while the others are all riddled with resentment having been relegated to the lower tier of the new Wu-Tang pay scale. This was once a team of unstoppable individuals united by a shared experience, but the Wu-Tang of 2014 is all cliques and alliances, communicating via their managers and magazine interviews.

All of these things matter, not least because the central themes of A Better Tomorrow are celebration, nostalgia and unity. RZA decided the Wu-Tang's destiny years ago and he's been stubborn enough to try and bend reality to fit his narrative arc. As if it wasn't going to be difficult enough to coax memorable bars out of these tired old farts – all well into their 40s – they're also toeing a party line that they don't necessarily agree with. This much is clear from the album's brilliantly-titled opener, 'Ruckus in B-Minor', as the Abbot attempts to galvanise the troops, hollering; "GZA! This is called Ruckus in B-Minor. Rae! All those bad times are behind us. Ghost! Put that mask on to remind us" – but none of those guys showed up to rap their verses when the group appeared on Letterman a week before release. It's that collective thrust which is completely absent throughout –and while one group-chanted chorus might claim "We live our lives tonight as one," you just don't believe it like you believed the Wu Tang Clan were nuttin' to fuck with.

RZA does his fair share of huffing and puffing on A Better Tomorrow (see hooks to 'Hold The Heater' and 'Crushed Egos'), but the widescreen production lacks the intensity to motivate a jaded clan. His chops as a musician may have measurably improved in recent years – he plays guitar, synths and arranges strings across the album's 15 songs – but as a producer of rap beats his powers have undoubtedly sagged. It's the difference between writing a movie score for Hollywood every so often, and locking yourself in a basement for five years cranking out the beats that defined an era. 

That's really what's sad about A Better Tomorrow, not that it's offensively bad for the most part, but that you're listening to decaying heroes. It's not surprising that they're creatively spent as a hip hop force, but to hear Inspectah Deck rapping about his favourite TV box sets on two separate songs is enough to kill any vibe. Method Man's flow might still be intact, but he's lost his cool – ordering young'uns to pull their pants up and asking us whether we still remember Tical. Ghostface falters more noticeably, frequently off the pace where he used to set it and struggling to make an impact like a killa bee without its sting. On 'Miracle' he takes a wild swing for his 'I Can't Go To Sleep' voice and misses by a distance, tossing off something vague about Ebola and the FDA. His verse is far from the song's lowest point though – Nathaniel's saccharine Disney hook is an abomination, while the climax sounds like what I imagine Linkin Park now sounds like. There's a decent enough Raekwon verse buried in there somewhere, but it can't save what will probably go down as the worst song Wu-Tang ever made.

It was much easier to fall for RZA's talk of growth around 8 Diagrams, but now we're here we can see it's been more of a cleansing than a progression. Recognisable voices and crackly kung-fu samples are just about the only thing connecting the Wu-Tang of today to '93 through '97, as the transformation to a live band sound has everything sounding crisp, polished and clean. 'Preacher's Daughter' interpolates 'Son Of A Preacher Man' and has members taking it in turns to, well, rap about the preacher's daughter – it's mix of bluesy guitar and Mark Ronson brass feeling like a direct pitch for Later... with Jools Holland. Singles 'Ron O'Neal' and 'Keep Watch' don't fare much better, the former notable for another disaster hook from that man Nathaniel ("No matter what the weather, we be gettin' that cheddar"), while Raekwon reviewed the latter himself, telling Rolling Stone: "I hate it. I hate it. I don't hate shit but I hate that fucking record."

The record's final chapter is a sentimental affair dominated by giant soul samples, bigger picture rhymes and an excerpt from Martin Luther King's 'I Have A Dream' speech – but the pay-off can't deliver when the journey has been so unconvincing. On The O'Jays-sampling closer 'Wu-Tang Reunion', Masta Killa raps about potato salad and kids playing Twister at a clan family BBQ in the future – but the Wu-Tang he's talking about is no more than idea. It's the same fairy-tale ending that RZA wrote into the script when he first conceived of A Better Tomorrow, and it's just as hopeful now as it was for ten kids growing up in the slums of shaolin.