Harrison Barnes has been in a month-long funk that's reached the point of disturbing the Spurs' offense. He may be shooting 38% from deep for the season, but he's only hit 30% of his attempts over the past month. He's had his moments, but overall, he's been very unreliable, so Mr. 100% has been hindering San Antonio's firepower.
Being off for a week or two is manageable. Teams don't overreact because they're still respecting the shooters. Cold streaks do happen after all. After a month, they start relaxing the coverage if you're going to keep throwing up bricks and opt for sending help to other places until that player can prove themselves a threat again.