When I charge my relatively new ISI cream dispenser, the N2O gas leaks around the top and body seam, resulting in failed whipped cream. I've tried everything to tighten the seam, including adjusting the gasket and tightening the top very hard. The demonstration videos show the cook lightly tightening the top and they seem to have no trouble. What am I doing wrong?
Is it clean? it does not take much pressure to properly seal an iSi. Either it is dirty or it is defective. I have had the issue you describe, the issue was not actually in the gasket around the top of the container, but instead in the gasket around the nozzle. It must be completely clean to hold pressure.
You need to know exactly which place the gas is leaking from. Charge it in a sink full of water and see where the bubbles escape from. Visually inspect the gasket and clean it, if it still leaks you'll need to get a replacement.