Hello @sammyla
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your sink plug.
If your sink plug is refusing to pop up, I suggest replacing the center post with a Kinetic Pop-Up Waste Mechanism. The old one can be removed by turning it anticlockwise. make sure to bring it to the store so that you can compare it to the Kinetic set to make sure they are the same size.
If you have any other questions we can help with, please let us know.
Eric
