The Mysteries of Orange Buddleja

I have two orange-flowered Buddleja (or Buddleia - apparently the 'j' form is the more correct) bushes in my garden,  both of which started out as cuttings from an ‘escapee’ growing in local woodland. This variety, is, according to Wikipedia, a hybrid called  Buddleja × weyeriana, the result of a cross between B. globosa × B. davidii. The latter is the purple variety commonly found on waste ground etc, and I avoided it because of its  vigorous growth. But the orange version seems to have a down side – possibly more than one.
Firstly, though its flowers are often visited by bees and flies etc, it does not seem to be well-patronised by butterflies. This is reflected to some extent by a RHS study, though it appears to be the size of the bush and number of flower-heads that matter most to Lepidoptera rather than the species of Buddleja. However, it is known that butterflies and many other insects can detect uv light, and may be guided by it when it comes to choosing plants for food or egg-laying (though scent may be a stronger stimunlus). It may be that B. x weyeriani is not selected because it is a relatively poor reflector of uv light - different species/colours of Buddleja apparently vary quite considerably in this respect.
My two orange Buddleja bushes are planted in different positions - one free-growing in rather poor, slightly acid soil, and the other in a large pot filled with bedding compost. The free-growing bush is thriving and flowering, the pot-bound one is not, though I suspect not because it is pot-bound since the free-growing one was originally in a much smaller pot with poor soil yet did very well. It's obvious that Buddleja species generally do well on poor soil, so my nutrient-rich compost is probably responsible for the difference in growth, though I haven't worked out why (there would be a clear reason in the case of leguminous plants, for example). The pot-bound bush has a companion - a passion-fruit plant (Passiflora caerulea)- which brings me to the second mystery of  B. x weyeriani: the passion-fruit is barely growing at all. The solution is clearly to re-home one or both of them, but I am curious as to why the passion-flower, a healthy, well-rooted plant from a really vigorous parent (sadly, not in my garden!) has fared so badly. I have wondered whether root allelopathy may be to blame, with the Buddleja producing chemicals that inhibit the growth of the passion-flower, however I can find no reference to this online. The free-growing plant doesn not seem to have affected the Hydrangea and Crocosmia plants near it, but that may be because those species are more resistent, or perhaps because any allelotoxins produced drain away more easily. Some experimentation is needed!