Can you post a pic? What is the tree planted in (the mix)? Most of us here fertilize regularly with Foliage Pro which seems to work wonders. I suggest you start by switching to that.

I would like to upload a photo but it isn't working, I might have to sign up to a photo posting website, upload the picture, then attach the link on here. I will look for foliage pro. I would love to correct the issue with the tree, I spent an hour and a half in the sun digging a huge hole in our clay garden then mixing in a bag of organic compost. Maybe the drainage is too poor or it has a disease, as it already was showing signs of yellow leaves in store, I was silly to buy a sick tree but a lady who worked at that garden centre recommended this tree, but now I know otherwise.