With tokamaks, there is a concept of confinement time. It turns out that plasma is not stable in a standard donut configuration and it likes to twist and break the continuous column, sending all the plasma shooting at the walls. The confinement time is how long it takes for this to happen. After the many plasma experiments over the decades, scientists have a really good idea of how the confinement time scales with magnetic field strength and volume of the plasma. It turns out that you want a big machine. With the JET tokamak they were really close. Iter is designed to have the confinement time well within the requirements for net positive energy, with like a 10-fold margin of safety. IMO it should work, but it is taking an unacceptably long time to construct.
If I recall correctly, the ITER project was very badly managed until 2015, resulting in the project being about 6 years late. A new management took over then, and the project is now moving on pretty swiftly. But indeed it's taking way too long...
