Generally speaking, a clam that doesn’t open is probably dead. While you could still eat it, don’t. You don’t know why it died (and there are lots of reasons…none of them good) before you got your hands on it, so don’t tempt fate. This also goes for open clams that don’t close when you tap them on the counter before cooking. As well, some clams are just so tightly joined to the shell that they’re not going to open—they’re alive but tough enough that you wouldn’t want to eat it anyway. Stubborn bast*rds!