In this chart, I can see that stable nuclides (other than hydrogen) have a neutron count greater than or equal to their proton count, and that the neutron:proton ratio for stable nuclides increases with atomic number.
I can also see that in many cases, radioactive nuclides which have excess neutrons undergo beta minus decay, converting one of the excess neutrons to a proton, and moving toward that stable ratio.
I can see why having excess protons might make an atom unstable: they have positive charge and push each other apart. But it seems to me that excess neutrons would make it more stable, adding to the strong force between the nucleons.
Why is an atom with excess neutrons unstable?