Unlike their heavier counterparts, the elements of Period 2, such as nitrogen and oxygen, participate in extensive pi bonding, often forming double and triple bonds. But heavier elements in the same group struggle to form any bonds past the first. Why is this?

This is due to the small size of group 2 elements, being much closer together when bonded. This allows the p orbitals of Period 2 elements to have significant perpendicular overlap, unlike those of heavier elements, which are much further apart. Heavier elements will thus form a longer and weaker pi bond, not as  energetically favorable, which explains why they prefer to only make single bonds.