One of the big problems with NERVA was that the T/W ratio of the engine alone was hardly above one, making it impossible to use for getting into orbit. Since the power of a nuclear reactor is directly proportional to its neutron flux, and adding more neutrons is probably not too difficult, why did we not see very high flux reactors used with the NERVA rockets?
Was plumbing the problem, or was power the problem?
The limiting factor on NERVA-type engines is the temperature of the core -- the more power produced, the hotter the core gets, and once the control rods start to melt or burn or otherwise erode, it's all over.
The "nuclear lightbulb" concept is one suggestion for achieving higher core temperatures (because the core doesn't have to remain solid) and thus more power, but there are many unsolved engineering issues with it.
In any case, open-cycle nuclear engines like NERVA would be inappropriate for launch from Earth's surface even if they had a high T/W ratio; because the reactor isn't sealed, radioactive material can escape with the exhaust, making for a severe health hazard.
