It's not a technical problem, but a bureaucratic dispute. NASA has been getting the Pu essentially for free, but the DoD/AEC made it as a byproduct.
Now they don't anymore, and would like NASA's budget to pay for its manufacturing.
NASA has decided it's not worth the money, and as a result is only using Pu power for missions that really need it, such as the large Mars rovers.
It's not a technical problem, but a bureaucratic dispute. NASA has been getting the Pu essentially for free, but the DoD/AEC made it as a byproduct.
Now they don't anymore, and would like NASA's budget to pay for its manufacturing.
NASA has decided it's not worth the money, and as a result is only using Pu power for missions that really need it, such as the large Mars rovers.