Welcome to the Bunnings Workshop community. It's splendid to have you join us, and thanks for sharing your question about your lemon tree.
This question is quite popular in the community and has been asked quite a few times. I suggest having a look at our Best Advice article Why is my lemon tree not making fruit? There are a number of possible causes and sometimes several of them could be afflicting the tree all at the same time. It can range from poor soil conditions, the root stock taking over and poor watering habits.
Here are some discussions regarding lemon and citrus trees: