Washer won't drain

All we did was move the washer across the room. Then it sat unused for about a month. Now it won’t drain. It will fill. It will spin. But it won’t drain.

We have:
Checked the lid switch.
Checked the hoses for kinks and twists.
Removed the hoses from the pump and checked for clogs.
Turned the pulley - it turns freely.
Looked into the pump as far as we can see.

We see a black rubber seal or valve in a closed position. Is this the problem? Is it not opening? If this is the problem, what makes it open? And how do we check it?

Thanks for any help.