Put a co2 capsule in and take up the slack with the knurled wheel under the capsule as usual.
But then only turn the piercing system about 15 degrees, dimpling but not piercing the top, then back off and take up the slack with the knurled wheel again before piercing fully.
If this cures the leak it's the capsule seal that needs replacing, or you can just keep doing it the way described above
ATB,
Nick