Ask Alys Fowler: will my magnolia ever flower?

Our gardening expert has the answer My Magnolia grandiflora has not flowered since it was planted 14 years ago, in clay soil near a well-behaved septic tank (ie, it’s not waterlogged). It is sheltered from the wind by a tall privet hedge 10ft away and is large and very healthy. Can I add something to the soil to help it flower?
Patience is required here. Bull bay magnolia takes around 12-15 years to flower. Yours may just be getting such a nice boost from the septic tank that it is concent