What happened to some of my Buddleia growing tips?

I have five Buddleia cuttings (taken last year) that have been growing extremely well this year, but some of the new growing tips developed a problem where the very tip of the growing shoot (and its tiny leaves) died, turned brown and the shoot stopped growing. The rest of the affected plants are fine. The growing shoot reached well over 12 inches in length before this happened.

As the cuttings are less than a year old there is, on average, three main new shoots per cutting.

This affected growing tips would have eventually formed flower buds (as the unaffected ones have done).

Sorry to say that I can't take any photos as I nipped out the dead growth (less than half an inch of growth). On average the new growth is about 18 to 25 inches long.

Any ideas what may have caused this growth tip problem please?

Thanks