Well at the moment we haven't built a reactor capable of sustaining Fusion for any useful length of time. JET (joint European Tokomak) is the largest MCF (Magnetic Containment Fusion) experiment at the moment but ITER is due to be built in France and is aiming to get a Q value of around 5 with a sustained plasma (that means that it will be able to kick out 5 times more energy then you need to put in via heating). But this still will not be a commercial reactor. The one after that (DEMO) is where they plan to get something that can viably produce power for the grid.

The main problem at the moment is plasma turbulence, as you heat the plasma up (to cause fusion) ELMs occur which are basically violent eruptions in the plasma, this causes the plasma make contact in parts of the machine that it shouldn't, this in turn knocks shit off the inner walls which fall into the plasma and cool it down (because these are larger ions so radiate energy away more quickly that the hydrogen). Cold Plasma = no Fusion.

There are other ways of doing fusion (inertial confinement) which I know nothing about so don't know why the're not working (think they don't have a focused enough laser or something).