Buddleia globosa problems?

Hi all, new here and hoping for an answer, ive bought about 3 of these in the past and they have all died - yes a buddleia of all things, I am north west so it can get cold in winter here, the latest most recent one I planted was in its second year and I thought I’d cracked it it had plenty of balls …. I will try to explain what happened as best I can without photos as I didn’t take any being so in sensed about the whole thing…. It flowered and them threw it more growth but as it did the green growth from the base of the plant was dying… this happened all over until it was bare ‍♀️…. The whole process finished about July maybe August time…. I searched all
Of Google to find reasons as to what happened and found nothing… I can only think a soil bourne pest? many thoughts would be much appreciated.

 

After B. globosa flowers it does put on new growth, which has the next season's flowers buds, dormant and not visible, at the apex of the stems in the autumn.
The species does tend to let some of the new growth die, particularly at the base, to concentrate the energy into the upper branches. The old leaves are deciduous, and are lost in the autumn, although as a whole the plant is mostly evergreen.
When there is much more foliage lost than kept, and where the new often growth fails, this is a sign it is not happy. The most likely problem is with the roots - either the soil is too wet, or too compacted for the roots to spread. Roots can also be damaged by windrock if in an exposed location. B. globosa also doesn't like to be contained, shouldn't be grown in shade, and can become hungry for nutrients. Wet is far more likely to kill it than cold, being hardy to about -15ºC.
It is probably too late to do anything about waterlogging. But you can aerate the soil around the base with a garden fork, which I find helps in heavy soil. Also, mulching with compost will both lighten the soil and provide some extra nutrition.
If you have to buy another, I would definitely look at whether the position is becoming waterlogged in the winter. Cold wet soil increases the risk of Phytophthora and Pythium root moulds infecting even resistant plants.