What are these cuttings growing in? If soil or planting mix, is it damp or dry? Does it smell bad at all? Any plant food? Hot sun? Pictures?

They are just in normal garden soil (mine is a little heavy and clay-like, but it's not waterlogged at all). Both plants (affected and unaffected) are in the same soil, only a few feet apart. The soil smells okay, no plant food used, afternoon sun only (not that hot in my part of the world (UK)).

Here's a photo of a tip of a shoot that has grown from a longer shoot which had the problem a month or so ago (as outlined in my first message) - you'll see that the problem is also present on this new shoot:

http://i59.photobucket.com/albums/g319/Dalek99999/P7301230.jpg

This will also eventually turn completely brown and die, just like the others did a while back.

So they aren't growing in a container? They are plants in the garden?

This kind of damage might be associated with too-wet soil, but if the soil isn't wet (and you have another identical plant right near it that is fine) then my guess would be that the roots have been damaged in some way - either because they were temporarily waterlogged in the past and rot set in, or physical damage occurred, or a pest or fungus compromised the roots. If there are no adverse conditions that you can correct, it's probably just one of those things and the plant may not make it.

Correct, they are in the soil, not in containers. Thinking about it that area of the soil DID get pretty waterlogged some months ago (the rainwater downpipe was, unknown to me at the time, missing the drainage opening pipe in the ground - now rectified).

However, what's curious is that the plant with the two unhealthy shoots/stems also has one that is extremely healthy, is flowering, putting out more flower buds and growth, etc. If it was a past waterlogging issue why would two stems on the same plant be affected but not the third?

Perhaps I should cut back the two affected stems really hard?

Clearly, it could be a lot of things, but Buddleas are known to be very sensitive to "wet feet." They simply die off quickly if their roots are too wet. You might have some root damage. If it continues you might dig up the plant and put it in lighter soil. Even if plants are very close to each other the conditions might be different in some respects.

Thanks very much, I'll keep an eye on it.

If I do transplant it when is the best time to do that? Winter?

I think you can transplant now. I'm not clear on your weather, however. Do you get freezes in the winter? If so, you don't want to transplant then. If you have a few months before very cold weather you should be able to transplant now. That would cut down on flowering in the short run, but it should allow the plant to root before it goes dormant--but again, I'm not clear on your weather.

Typical UK weather here, so quite a micture really. Yes, we get frosts in most winters.

A thought though - as one shoot of the plant is very healthy (and two aren't) does that imply that there's only SOME root damage and that, if left where it is (and no waterlogging), new root growth will ensure good shoot growth next year?