Generally due to a lack of on/off system? If you use a standard asa you are putting a lot of pressure on the threads of both the asa and the bottle each time you attatch / detach the bottle while pressurised.Bush said:
When I got my impy back in 2004 I had no on/off. Towards the end of a walk on I realised that I could not remove the bottle. In the end it took 2 of us. One holding the marker the other holding the reg with a monkey wrench. We got it off but wrecked the regulator / asa threads. I then had to spend £60 buying a new reg and on/off system.