My 9 year old olive tree has only fruited once in 9 years. I wonder why it is perfectly healthy

Most olive trees are self-fertile or self-pollinating, so they can fruit without the need for another tree. However, fruit production can be greatly improved with the presence of another tree, especially when conditions for self-pollination are not optimal. If possible, you may want to consider planting another tree nearby. 

It's also a good idea to feed your tree regularly to ensure it is healthy. Try Yates Thrive Natural Citrus & Fruit Organic Based Pelletised Plant Food. Click here for more information: