Long handrails on decks and porches tend to sag in the middle, especially after they've been subjected to the weight of people sitting on them. A wood prop block made from a pressure-treated cedar or redwood 2x6 will shore up a sagging rail and keep it level.
Start by measuring the distance between the decking and the underside of the railing at one of the posts. Cut the block to that length. Then use a couple of 2x4s as a lever and fulcrum to raise the railing enough to slide the prop block into position. Remove the lever and screw or nail the block to the decking and railing.