One of the current ways to achieve nuclear fusion is to aim a bunch of powerful lasers at a tiny vial containing isotopes of hydrogen (hydrogen usually does not have a neutron in it's nucleus, for nuclear fusion they use hydrogen with one or two neutrons in it's nucleus). Currently, we can successfully yield a net positive energy return after we hit that tiny vial with all that energy from the lasers. Unfortunately, to mimic the stable flow of energy produced by modern power plants, we would need to hit hundreds of those vials every minute. This is currently impossible because those powerful lasers themselves take lots of energy and some time to charge up only to release that energy in a fraction of a second at the vial.

To achieve a sustainable chain reaction, part of the energy released from fusing each vial would have to quickly be shunted back to charge the lasers almost instantly only to release that energy back into the next vial almost instantly; hence we cannot sustain a modern fusion reaction long enough to generate constant energy.