At a packed London club, Teenage Fanclub warmed up for tonight’s Celtic Connections show in Glasgow with a good-humoured set full of mellifluous, folk-tinged retro-rock. Norman Blake, one of the Bellshill quartet’s three singing guitarists, was in a typically chirpy and self-deprecating mood.

Paying scant regard to musical or critical fashion, the so-called Bellshill Beach Boys have stuck with the same basic songwriting ingredients during almost 20 years together. All chiming guitars and keening vocal harmonies, their sound remains firmly rooted in the West Coast rock scene of 1960s America. This consistency is both a strength and a weakness.

Few bands ever took premature adulation in their stride with quite as much charming indifference as the Fanclub. In 1991, their landmark Britgrunge masterwork Bandwagonesque earned hysterical rave reviews and a place in the modern rock pantheon. Soon afterwards, their Creation Records labelmate Liam Gallagher called them “the second-best band in the world” – after Oasis, of course.

Yet less than a decade later these stoic Scots found themselves without a record contract. Naturally, they plodded on regardless. They released their most recent album, Man-Made, on their own PeMa label in 2005.

At their best, Teenage Fanclub’s songs embody a simple beauty bordering on the transcendent. Among the highlights in London was their 1997 single Ain’t That Enough, which pays awestruck homage to the natural majesty of sunrises and clear skies. Imagine Thoreau’s Walden set to soaring, jangling guitars.

The drawback of such laid-back understatement, of course, is that it shades all too easily into complacency. In his book 31 Songs, Nick Hornby rightly praises Ain’t That Enough for its comforting and unchallenging beauty. But such loveliness is clearly double-edged. No man can live on sweetly sparkling guitars and creamy vocal harmonies alone. Much of the London set certainly lacked emotional bite.

Many tracks, including I Need Direction and Mellow Gold, sounded sluggish and sleepy. Restrained and rustic in mood, Teenage Fanclub shows invariably radiate midlife contentment nowadays. This is pleasant enough in small doses, but it undersells their more melancholy and sardonic compositions.

Perhaps inevitably, the songs from Bandwagonesque packed the most punch. The rowdy guitar blasts of Star Sign sounded a little undercooked, but the honeyed headbanger chug of The Concept helped to boost flagging excitement levels in the show’s latter stages. Alas, perhaps because the band played the album in its entirety 18 months ago, as part of the Don’t Look Back concert series, they dipped into it only sparingly on Thursday.

Nobody can blame Teenage Fanclub for sticking to their musical comfort zone during two decades of fluctuating fortunes. But a dash more passion and grit would serve these finely crafted tunes better, as would less reverence for such a narrow strand of rock history. We all know they are world-class trad-rock songwriters. They just might benefit from challenging their audiences, and themselves, a little more.

Teenage Fanclub play the Old Fruitmarket in Glasgow tonight (0141-353 8000)

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