IIRC, "Viktor Suvorov" (real name: Vladimir Rezun), the defector who wrote several good books about the Soviet Army, stated in the 1980s that the one thing that limited the USSR's strategic arsenal was an acute shortage of fissionable material, and that a lot of warheads were loaded with radioactive waste instead of plutonium.
They might be missing the same component--the important one...
The above is my "gut read" on the topic.