Water problem, probably - if the pot's not very big and the plant is in sun, there's not much compost to hold the water, so it will need watering thoroughly, probably a litre in the evenings. Turn it out of the pot and check the rootball - if its bone dry, put it back in the pot, stand the pot on a tray and pour in 2 litres of water or more, till its running out the bottom and collecting in the tray, and let it sit and soak for a couple of hours, then take the tray away. And I'd just add that, unless your buddleia is a dwarf variety, they don't do well in pots unless the pots are absolutely huge.