SpriteKit only detecting partial collision on physics body - objective-c

Question

I have a simple jumping game setup where an hero jumps over a ball rolling towards him.
there is no gravity setup on the scene
the hero jump straight up and then straight down. if the hero stays still "didBeginContact" is fired and I can detect the collision. If the front of the hero hits the rolling ball when jumping up or coming down "didBeginContact" is fired. However if the back half of the hero land on the ball as it is going under it, it does not detect the collision.
Does anyone have any idea why it only catches the collision on the front? This seems more like edge detection vs. object detection! is that possible?
static const uint32_t groundCategory = 0x1 << 1;
static const uint32_t obstacleCategory = 0x1 << 2;
static const uint32_t heroCategory = 0x1 << 3;
SKSpriteNode *hero = [[SKSpriteNode alloc] initWithColor:[SKColor redColor] size:CGSizeMake(100, 115)];
[hero setPosition:CGPointMake(HERO_UP_POSITION_X, HEIGHT_GAME / 2 + hero.size.height / 2 + 10)];
hero.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:hero.size];
[hero.physicsBody setAffectedByGravity:NO];
hero.physicsBody.usesPreciseCollisionDetection = YES;
hero.physicsBody.allowsRotation = NO;
hero.physicsBody.categoryBitMask = heroCategory;
hero.physicsBody.contactTestBitMask = obstacleCategory | groundCategory;
hero.physicsBody.collisionBitMask = 0;
hero.zPosition = 0;
hero.name = kHeroUpName;
[self addChild:hero];
obstacle = [[SKSpriteNode alloc] initWithImageNamed:kImgObstacleDown];
obstacle.name = #"obstacle";
[obstacle setPosition:CGPointMake(WIDTH_GAME + x, HEIGHT_GAME / 2 + y)];
obstacle.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:obstacle.size.width / 2];
obstacle.physicsBody.usesPreciseCollisionDetection = YES;
[obstacle.physicsBody setAffectedByGravity:NO];
obstacle.physicsBody.categoryBitMask = obstacleCategory;
obstacle.physicsBody.contactTestBitMask = heroCategory;
obstacle.physicsBody.collisionBitMask = 0;
obstacle.zPosition = 1;

Answer 1

I figured out the problem.
I was checking if the object was past the hero in order to track score, If it was past the center of the hero I renamed it, so that it wouldn't get checked and scored again.
Thanks #0x141E your snippet made me realize that I was checking for names inDidBeginContact (Which fails) vs. checking for categoryBitMask

Answer 2

There are two test cases that needs to be in your didBeginContact: 1) if contact.bodyA is the hero and contact.bodyB is the obstacle and 2) if contact.bodyA is the obstacle and contact.bodyB is the hero. You are likely handling only one case. Here's an example of how to do that:
-(void)didBeginContact:(SKPhysicsContact *)contact
{
if ((contact.bodyA.categoryBitMask == heroCategory && contact.bodyB.categoryBitMask obstacleCategory)
|| (contact.bodyA.categoryBitMask == obstacleCategory && contact.bodyB.categoryBitMask == heroCategory)) {
// Do something here
}
}