data-mm-id=”_jqxh8ju6s”>The NBA has introduced coach's challenges for the first time this season. So far, it hasn't gone terribly well, as they've run into the same problem that the NFL has — referees are extremely reluctant to overturn the initial call on the floor, even in the face of overwhelming evidence. I wrote about this problem back in November. Things haven't changed. Calls are rarely being overturned and coaches are getting aggravated. It's even getting Brad Stevens mad, and he's as even-keeled as they come. On Sunday, the Celtics played the Knicks. During the contest, Julius Randle made a bucket over Grant Williams, who was called for a blocking foul on the play. Stevens challenged the blocking foul with his coach's challenge and was right…