I have this here code snippet in the update method for a custom tile entity:

 

if(flag&&rand.nextDouble()>0.8&worldObj.canSeeSky(pos.up())){
if(!worldObj.isRemote){
	System.out.println("TEST");
	EntityTNTPrimed f=new EntityTNTPrimed(this.worldObj);
	f.setVelocity(0,-3,0);
	f.fuse=120;
	f.setPosition(pos.getX()+rand.nextDouble()-0.5,250,pos.getX()+rand.nextDouble()-0.5);
	worldObj.spawnEntityInWorld(f);
}
}

 

(flag being a custom boolean field indicating if the block has ever detected a redstone signal since having been placed)

 

I know from reading the console output that this block of code is being executed, but the TNT entities are consistently refusing to spawn. Why is this?

take a look how minecraft spawns tnt (BlockTNT)