Have you applied a vine weevil drench - the grubs could be eating your roots and causing the damage? Hence one pot is okay but the other is not? If you tip out the pot and find small curled up white/cream grubs then you have vine weevil. Do not remove the soil to your compost or other areas until it has been treated, and water the medication around freely!

This may not be your problem, but will eliminate it from the list!!

Thanks for your help. I'll give that a try.

I applied a vine weevil drench to both the star jasmine & the bay.

I think the start jasmine probably did have vine weevil due to the notches out of the leaves - I've loaded up a picture. It looks like it isn't going to make it. There doesn't seem to be any new growth and it's lost most of the leaves since the picture. It hasn't quite gone yet so got my fingers crossed but I suspect the vine weevil may have done too much damage before I go to it.

The bay tree (picture loaded up) is a much bigger and more established plant and doesn't seem to have changed at all. Not sure if vine weevil is the problem here. it isn't really dropping any leaves but not got any new growth either. I have tried giving it a feed to promote leafy growth as well but nothing is happening. Any other ideas on what could be wrong with this one?

Thanks for any help you can give.