Newly planted Buddleia leaves going yellow!

I recently created a double-wall, filled the gap with ordinary topsoil, then a weed-suppressing membrane and finally some bark chippings.

After things had settled I planted a row of Buddleia all but one of which are turning yellow (see photo). I cannot see any bugs. The plants were healthy when bought (about 2 months ago) and they came from different suppliers so unlikely to have root issues.

There has been rain, but not overmuch, and I did keep them watered (but not flooded) after they were planted. The wall does have drainage.

I don't have a soil testing kit, but it was jumbo bags of local topsoil and looked normal.

Can you recommend a fertiliser or supplement to help them get established?