SAN DIEGO — Courtney Porter, 23, sat on a wall in front of the San Diego Convention Center last week with a backpack and carry-on suitcase. He looked dejected while flipping through a sheaf of papers that listed several local homeless shelters. A phone call to each one resulted in the same answer: “I’m sorry, we’re full.”

Porter arrived a few weeks ago from St. Louis, but his unemployment check and hawking an iPad haven’t been enough to pay for rent. Desperate, Porter showed up at one facility and waited on an adjacent patio.