What they lagged on was the engineering expertise to put the theories into practice. The US spent billions to develop this expertise in a short period. The Nazis simply did not have the resources to spend on this development. What little they put aside for new weapons development was spent on jet fighters and the V1/V2 missiles.
They were not even close to developing an A bomb. They had no equipment for seperating U235 on an industrial scale, no reactors for plutonium production, no research facilities for determining the critical masses necessary (interesting stories from Los Alamos on "tickling the tiger" from a physics prof I had), etc.
Yes. Obtaining sufficient fissionable material and constructing it in a way where it explodes on command and not before, are still major hurdles for countries like Iran.
Feyman, I believe.