I can recommend a great article answering that, however unfortunately it's in German [1].
But the gist is that there are two major unsolved challenges.
One is finding a material stable enough to let the reactor run for longer times (all experiments like ITER only plan to run the reaction for very short times, which is okay for research, but not useful for real energy).
The other is finding a way to produce enough tritium for the reaction. It is expected that ITER alone will consume all the tritium that is available and that is currently only produced in some special kind of nuclear reactor. The idea is that eventually fusion reactors will create this tritium themselve, but noone has figured out how to practically do that.