For controlled thermonuclear fusion to work, you need to confine an extremely hot plasma (ionized gas). It needs to be hot enough and dense enough for the fusion reactions to initiate and proceed. It is extremely difficult to get the plasma to the conditions needed for thermonuclear fusion to start, and then you have to keep it at those conditions long enough to get significant energy output. So far, plasma confinement has been very difficult. Many of the attempts to confine plasmas with extremely strong magnetic fields have failed. Laser fusion methods have shown some potential, and are being investigated at high energy laser experimental facilities. But we are not yet at the point of "igniting" a confined plasma to produce energy.