Woolly aphid problems
It could be a variety of pests that are attacking your foxgloves. If the leaves have lots of little holes this could be the result of Flea beetles. Aphids will secrete a honeydew material on the plant and weaken the plant. Both of these pests and Woolly aphids can be controlled by spraying a natural pyrethrum insecticide such as Insect Killer spray from Harrod Horticultural. It kills the insect pest on contact, so good coverage is required. However if the foxgloves are in flower try to avoid spraying the flowers as the spray can damage them. Woolly aphid often need several sprays to break down the waxy material and kill the insect underneath. The other option is to apply Ladybird larvae to plants with woolly aphid, which Harrod can also supply.
Hope this helps.