Question
I have had 5 Jasmine plants in brick planters, facing south, for 4 years and they only flowered the first two and have got steadily weaker. They sit with ivy, rosemary, thyme and a dwarf conifer (The supplier had just this arrangement and it looked fine) They are still alive and there are some new shoots but generally, the leaves are sparse and go dark red in winter. There is also some grey/black dust on the backs of a lot of the leaves.

I've treated blackfly and ants as they came along and have put in plant food and always make sure they are watered.

The planters are topped with tree bark. The other plants all do exceptionally well though the thyme seems to be crowded out a bit now. The Jasmine climbs the wall with the aid of trellis to 6 feet and is trained along a horizontal wire at that height.

We live in North Wales, UK.

Any help to make these specimens flower and thrive again would be most welcome.

Answer
Your environment may be a little harsh for jasmine.  The red leaves in the winter is an indication of being a little too cold for those plants.  Try cutting them back severely and see if they will sprout out new more vigorous growth.  The root systems are obviously compromised and are not able to keep up with the top growth. Continue to do everything else you are doing and see if they improve this year.  Thanks.