Gold star magnolia

We planted a gold star magnolia tree in a very large pot in the garden in April as a memorial to my son Gavin who died last November. we placed his ashes in the pot under the plant and used high quality compost for planting.   Although the tree hasn't died, neither has it flourished. It hasn't grown much and Im not sure wether its just the time of year, or wether its what its planted in. Any idea what I can do to help it to flourish for next spring? regards Carole