Why am I getting runny whipped cream from my iSi canister?

Things to check:

Seal. If the seal is broken/erroded gas will be able to escape and will therefore not whip your cream - giving you a liquid.

Canister (Charge) - Are you using a fresh charge (ie. NOS canister)? While it is unlikely, it is possible that you have either a dud box or it has somehow become damaged.

Position - Are you holding it correctly? I believe it should be completely inverted when using it forcing the gas through the cream as it escapes. Double check your instructions for your specific whipper.

Cold cream will work better than warm - What is the temperature of the cream you are trying to whip? Also I assume you are using 100% dairy cream? I've had mixed results with things like elmlea (a UK low fat "cream" that uses vegetable oil instead of dairy fat).

If you read the manual for the ISI whip, it has a few suggestions.The list goes like this:

-The whip is overfilled, pour some out and repressurize

-The device was not shaken enough (shake it vigorously and try again)

-The contents are too hot (let cool in the fridge)

-Your cream contains too little fat (it needs to be 30% minimum)

-If using a binding agent, you may have added the improper amount or used it incorrectly.

http://www.isinorthamerica.com/fileadmin/files/images/iSi_North_America/Images/Documents/Creative_Whip_ENGLISH.pdf

A cold canister held upside down works best.

I double charge my canister and find it produces a better structured, denser cream. I haven't read about doing this anywhere else so maybe I'm going to blow myself up one day.

I also like to flavour my cream right in the canister. A packet of good vanilla sugar does wonders. I know some chefs heat the cream first to melt in a little white chocolate and then cool it and put it into the canister (haven't tried this yet).