En mi juego tengo 4 máscaras de bits y todo está configurado, pero didBeginContact solo se llama cuando la primera máscara de bits (categoría de jugador) choca con algo. si 3 colisiona con 4, no sucede nada, aunque tengo el contactTestBitMask configurado para que colisionen.

myscene.h

    self.physicsWorld.gravity = CGVectorMake(0.0, -2.45);
    self.physicsWorld.contactDelegate = self;

    self.player = [SKSpriteNode spriteNodeWithColor:[SKColor blueColor] size: CGSizeMake(10.0, 20.0)];
    self.player.position = CGPointMake(20.0, self.frame.size.height);
    self.player.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:self.player.size];
    self.player.physicsBody.affectedByGravity = YES;
    self.player.physicsBody.dynamic = YES;
    self.player.physicsBody.categoryBitMask = playerCategory;
    self.player.physicsBody.contactTestBitMask = enemyCategory;
    self.player.physicsBody.collisionBitMask = tileCategory;
    [self.gameNode addChild:self.player];

    [...]

- (void) swipeRightHandler:(UISwipeGestureRecognizer *) recognizer {
    NSLog(@"swipe right");
    SKSpriteNode *attackRect = [SKSpriteNode spriteNodeWithColor:[SKColor redColor] size:CGSizeMake(40, 5)];
    attackRect.position = CGPointMake(self.player.position.x + 10, self.player.position.y);
    attackRect.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:attackRect.size];
    attackRect.physicsBody.categoryBitMask = attackCategory;
    attackRect.physicsBody.contactTestBitMask = 255;
    attackRect.physicsBody.collisionBitMask = enemyCategory;
    attackRect.physicsBody.affectedByGravity = NO;
    attackRect.physicsBody.dynamic = NO;
    [self.gameNode addChild:attackRect];
    [attackRect runAction:[SKAction moveBy:CGVectorMake(250, 0) duration:1.0] completion:^{
        [attackRect removeFromParent];
    }];
}

RandomLevelGenerator.m:

        SKSpriteNode *enemy = [SKSpriteNode spriteNodeWithColor:[SKColor greenColor] size:CGSizeMake(20, 20)];
        enemy.position = CGPointMake(visual_x, FLOOR_X + arc4random_uniform(MAX_Y - FLOOR_X));
        enemy.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:enemy.size];
        enemy.physicsBody.categoryBitMask = enemyCategory;
        enemy.physicsBody.contactTestBitMask = playerCategory | attackCategory;
        enemy.physicsBody.collisionBitMask = attackCategory;
        enemy.physicsBody.affectedByGravity = NO;
        enemy.physicsBody.dynamic = NO;
        [self.scene.gameNode addChild:enemy];

LevelGenerator.h:

static const uint32_t playerCategory    =   0x1 << 0;
static const uint32_t tileCategory      =   0x1 << 1;
static const uint32_t enemyCategory     =   0x1 << 2;
static const uint32_t attackCategory    =   0x1 << 3;

MyScene.m nuevamente:

SKPhysicsBody *firstBody, *secondBody;

if(contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask)
{
    firstBody = contact.bodyA;
    secondBody = contact.bodyB;
} else {
    firstBody = contact.bodyB;
    secondBody = contact.bodyA;
}
NSLog(@"%d - %d", firstBody.categoryBitMask, secondBody.categoryBitMask);

El último NSLog solo imprime 1 - x, nunca 3 - x. También he intentado permitir otras colisiones y no consigo que ninguna de ellas choque.