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).