Buddleia bushes - how to make them flower for a lot longer?

I have a few Buddleia bushes of different colours (pink, white, purple, crimson) that are a few years old. Many are of moderate to large size and all of them flower very well but the flowers quickly go over and not that many new ones are produced afterwards, they only flower for maybe two weeks. All are in different areas of my garden with differing amounts of sunlight (some get direct sun most of the day, others get direct sun only in the afternoon).

I know about dead-heading to encourage new flowers but have tried it with no success.

There are also other Buddleia bushes in the hedgerows in my general area (mostly the pink ones, some white) and all of them, without fail, flower for a lot longer than mine. None of these are dead-headed either of course.

Might it be the soil type causing my issues? It's a fairly clay-like soil. Or a nutrient issue?

Thanks

 

My Buddleia has flowered all summer and is still flowering. All we do is deadhead it.
I think the ones that grow in the hedgerows have not been bred for showy flowers and as such just want to produce flowers, seed and survive.

 

@TheMadHedger You could could look at some more modern varieties that might have longer flowering periods.
Like Jocko's mine go on for quite a while on just dead heading. I don't think it's likely to be a nutrient or soil issue. M B davidii gets hard pruned down to a foot to two foot either in autumn or spring and this does affect flowering time, spring pruning tends to give later flowering time, length of flowering.not affected.
If you want to be radical have a look at some of the different species I have Buddleja in flower most of the year round. B colvilei flowers in spring (red flowers), B weyeriana orange balls of flower most of summer, B officinalis flowers late winter/early spring, B salvifolia flowers in spring strongly scented.
Some can be difficult to find see here ABC | Farmyard Nurseries
and https://www.burncoose.co.uk/site/plants.cfm?pl_id=596

 

The funny thing is, my pink ones were taken from one growing in a hedge and that bush always bloomed for a long time.