This can be a result of the "O" ring on spout being perished, or it has been cut.
This is most often caused by pushing the spout into the main body before opening the grub screw fully at the back of the spout.
You may need to replace the "O" ring.
To ensure the spout has been pushed into the main body fully, unscrew the grub screw fully, and ensure the spout is pushed fully into the main body to form a double seal. Please then re tighten the grub screw.