I think it has been a case of trying to tdentify a laboratory method which preents some promise of steady progress toward a controlled, sustainable reaction. (I don't think it's a matter of more money.)
I don't see how you can know in advance whether a method will work until you spend some time with it. That's why certain methods (e.g. diodes) were abandoned.
It appears to be a problem whose solution can only be obtained through painstaking laboratory evolution. I personally am confident that the class of nuclear engineers working this problem are bringing the physics along as quickly as knowledge will allow.
Oops! preents=presents
There are too few working on the problem. That is because there are too few projects being funded. If we want to make some serious progress we need to employ more thinkers. A $ billion a year might sound like a lot, but for a problem like this, where they are at the edges of science and materials engineering all the time, it is laughable.