Make sure that you have put the cylinder on tightly enough. If you just screw it on till the valve opens and 'grabs' the cylinder,it will not maintain pressure and give you low power shots. Don't over tighten, but it needs to be nipped up hand tight.

If you need to reseal and can do it yourself try here:http://www.altecweb.com/home.asp?cat=RangeB05