Welcome to the Bunnings Workshop community @sheila. Many thanks for your question, I'd be happy to assist.
The doorknob fixes to the door by turning the golden coloured collar flange clockwise. The thread on it matches up with the thread on the door fixing and tightens it onto the door. If it does not tighten, then there is a chance the thread is damaged and it will need to be replaced. Unless we can decipher which thread is damaged then you might need to replace the whole handle set.
Please let me know if you need further advice or had questions.