"Come" question

We've been working with our 8mo pup on "come," and he's doing alright. The problem is that there are times, such as when he's relaxing or tired, that he just can't be bothered. We call, treat in hand at times, and he lies there. We gently tug the leash...nothing but a blank stare. I've even gone over to coax him up physically and he becomes a sack of bricks.

So, I'm just wondering where to go from here.

Thanks!