What Causes a Nuclear Fizzle in Chain Reactions?

[Kronsteen]

The first tests of North-Korean nuclear bombs were reported to be "just a nuclear fizzle".
What does that mean?
Was the reaction speed to slow to create an explosion?
Where is the difference between an efficient chain reaction (resulting in an explosion) an a fizzle?

Thanks for your help!

 

[bcrowell]

Dunno for sure, but I'd suspect that it refers to a case where a chain reaction is initiated, but the fissionables blow apart before the reaction can run to completion.