The shell was probably a "binary" round rather then just a container filled with the lethal agent. Binary munitions have two compartments; each is filled with a relatively harmless "precursor" chemical. The two ingredients need to be properly mixed to form the finished nerve agent. This makes for much safer storage and handling since the shells don't become lethal until fired. Upon firing the round is "spun up" by the rifling in the gun barrel which causes the two components to mix properly. The bursting charge then releases and disperses the agent.
The terrorist who fabricated the "IED" more then likely did not know that he had a chemical weapon shell. Thinking it was an ordinary HE round, he wired it as a roadside bobby-trap. When the shell exploded very little actual Sarin was produced because of the omitted mixing phase.
That's most likely why it seemed to be a "dud".
Regards,
GtG
Now he and his terrorist buddies know. The implications are chilling. And the dominant news stories today are...?