When cooking clams you should kill them first by inserting a knife into the clam and cut the muscles and nerves to kill the clam so it cannot close back up.
Then when you cook the clams they should indeed open up when they're fully cooked.
When the clam hasn't opened up then the clam hasn't been cooked long enough or good enough so it means the clam is under cooked.
Eating under cooked clams can make you sick so make sure the clams are fully cooked and done.