I've seen this issue before for earlier versions, but none of the solutions given seem to have worked for me.

 

Basically the mob will run up to its target as if to attack, but it won't do any actual damage in survival mode.

 

Entity code:

public class EntityVampire extends EntityCreature{
    
	public EntityVampire(World worldIn) {
		super(worldIn);
		this.setSize(0.6F, 1.95F);
	}
	
	@Override
	protected void applyEntityAttributes()
    {
        super.applyEntityAttributes();
        this.getEntityAttribute(SharedMonsterAttributes.MAX_HEALTH).setBaseValue(20.0D);
        this.getEntityAttribute(SharedMonsterAttributes.FOLLOW_RANGE).setBaseValue(20.0D);
        this.getEntityAttribute(SharedMonsterAttributes.MOVEMENT_SPEED).setBaseValue(0.40D);
        this.getEntityAttribute(SharedMonsterAttributes.ARMOR).setBaseValue(40.0D);
        
        this.getAttributeMap().registerAttribute(SPAWN_REINFORCEMENTS_CHANCE).setBaseValue(this.rand.nextDouble() * 0.5);
        
        getAttributeMap().registerAttribute(SharedMonsterAttributes.ATTACK_DAMAGE);
        this.getEntityAttribute(SharedMonsterAttributes.ATTACK_DAMAGE).setBaseValue(10.0D);
        getAttributeMap().registerAttribute(SharedMonsterAttributes.ATTACK_SPEED);
        this.getEntityAttribute(SharedMonsterAttributes.ATTACK_SPEED).setBaseValue(2.0D);
    }
	
	@Override
	protected void initEntityAI()
    {
        this.tasks.addTask(0, new EntityAISwimming(this));
        this.tasks.addTask(2, new EntityAIAttackMelee(this, 1.0D, false));
        this.tasks.addTask(5, new EntityAIMoveTowardsRestriction(this, 1.0D));
        this.tasks.addTask(7, new EntityAIWanderAvoidWater(this, 1.0D));
        this.tasks.addTask(8, new EntityAIWatchClosest(this, EntityPlayer.class, 8.0F));
        
        this.tasks.addTask(8, new EntityAILookIdle(this));
        this.applyEntityAI();
    }
	
	
	protected void applyEntityAI()
    {
        this.tasks.addTask(6, new EntityAIMoveThroughVillage(this, 1.0D, false));
        this.targetTasks.addTask(1, new EntityAIHurtByTarget(this, true, new Class[] {EntityPigZombie.class}));
        this.targetTasks.addTask(2, new EntityAINearestAttackableTarget<EntityPlayer>(this, EntityPlayer.class, false));
        this.targetTasks.addTask(3, new EntityAINearestAttackableTarget<EntityVillager>(this, EntityVillager.class, false));
        this.targetTasks.addTask(3, new EntityAINearestAttackableTarget<EntityIronGolem>(this, EntityIronGolem.class, true));}
    
	@Override
	protected void entityInit() {
		super.entityInit();
	}
	

 

You usually need to override the attackEntityAsMob() method as well as the attackEntityFrom(), and possibly the canAttackClass() and the attackable() methods. These are inherited from EntityLivingBase and such, but for EntityCreature (which you have extended) they are not fully implemented. 

I overrode all of those methods and keep getting the same issue, so either I implemented them incorrectly or am forgetting something else.

 

Code:

	@Override
	public boolean attackEntityAsMob(Entity entityIn)
    {
        boolean flag = super.attackEntityAsMob(entityIn);

        if (flag)
        {
            float f = this.world.getDifficultyForLocation(new BlockPos(this)).getAdditionalDifficulty();

            if (this.getHeldItemMainhand().isEmpty() && this.isBurning() && this.rand.nextFloat() < f * 0.3F)
            {
                entityIn.setFire(2 * (int)f);
            }
        }

        return flag;
    }
	
	@Override
	public boolean attackEntityFrom(DamageSource source, float amount){//add zombie reinforcements
		if (source == DamageSource.DROWN || source == DamageSource.FALL){
			return false;
		}
		else{
			return super.attackEntityFrom(source, amount);
		}
	}
	
	@Override
	public boolean canAttackClass(Class<? extends EntityLivingBase> entityClass){
		if (entityClass == EntityZombie.class || entityClass == EntityVampire.class){
			return false;
		}
		else{
			return super.canAttackClass(entityClass);
		}
	}
	
	@Override
	public boolean attackable(){
		return true;
	}
	

 

Well, the next step in debugging is to trace the execution to confirm where it is failing to act as expected. So I use console print statements (System.out.println()) although you can also use debugging mode or other techniques for visibility during execution.

 

You'll want to ensure that each method is being called as you expect, so at the beginning of each put in a print statement. Then for any conditional statements (like if statements) I usually put a statement before to print out the values of the fields that will be tested and I also put a print statement in each path of the execution to confirm what it decides to do.

 

With that visibility into the execution it is usually very quickly obvious what is going wrong. The method may not even be called (maybe the AI is failing to cause an actual attack), or maybe you'll find some other problem.

 

For debugging entity AI, I often make may own copies of the AI classes that include print statements in the same manner as I describe above. Then it all becomes very clear. Maybe it isn't targeting, etc.

I figured it out...

 

It's because I extended EntityCreature rather than EntityMob. The tutorial I followed (yours, btw, I found it very useful!), having been written in 1.7.10, seems to be a little outdated in that regard.