Drain Test Bag Leaking

[Zippymark]

I've been asked to perform a pressure test on a soil stack i've recently installed for a new build and i'm using an inflatable test bag for the bottom and i cannot get it to seal. I flushed the toilet thinking maybe a bit of water on top might help it and i can see it dripping though. The bag said to inflate to 1bar, which i did, i even tentatively went up to 3bar and it still wouldn't seal. It was on a straight bit of pipe. Any ideas?

 

[ShaunCorbs]

Bit of silicon grease on the bag normally helps