Q.magnolia stellata in a large tub
I have a magnolia stellata in a large tub with the correct compost. It was planted two years ago and early this year it flowered but not as prolific as expected. Since then it has started to die. After some time, a few very small leaves started to grow on one side of the tree only but much of the previous growth has died off and gone brown with no signs of life. It is in a partly shaded position and I have applied slow release feed but without any improvement. How can I bring it back to life?

Magnolia Stellata is an excellent tree for growing in pots.
I do think what your describing is environmental. Did you have an extreme winter and cold? Any late frosts?
I'm sure you have inspected for insect damage. You could spray with an insecticide just to be safe. Your application of slow release fertilizer is a good choice. Continue to maintain it's care with proper sunlight and watering. Plants sometimes surprise us.
I think your at a wait and see crossroads.
Here is a link to refresh you of the care requirements.
https://www.gardeningknowhow.com/ornamental/trees/magnolia/caring-for-star-magnolia-trees.htm