This is usually the result of the "O" ring becoming corroded or damaged and is most often caused by pushing the spout into the main body before opening the grub screw fully at the back of the spout.
Following the below instructions to remove the spout, inspect the washer and replace it should it show any signs of wear or damage or if you re-tighten the spout and the issue continues.
To ensure the spout has been pushed into the main body fully:
- unscrew the grub screw at the back of the spout
- remove the spout
- inspect the washer and replace if needed
- push the spout down fully into the main body to form a double seal
- re-tighten the grub screw.