My bet: They don't know how to do the initiator. Without that, all you get is a dirty conventional explosion. Bad, but not as bad as if the initiator works and supplies the requisite neutrons to begin the chain reaction. Without those neutrons, at the correct energy, there is no chain reaction before the whole assembly blows itself apart.
It's not an easy problem to solve, which is why it remains secret. This, for very good reason.