The Paypal account for item payments is held in THE LISTING not in My Ebay > Account (which is the one to pay selling fees)

The problem is almost certainly a typo or old Paypal Acct in the listings themselves.

Go to a completed listing and choose "Relist" - scroll down and check the Paypal Address that is nominated - that is where the money has gone. (cancel the relisting process after checking this).

If you still have access to that email address, then go to Paypal > Profile and add it in there. Once you verify the email (reply to a test message), the money will automatically come through.

If you do not have access to the email address, then you have to phone Paypal Customer Service and explain what has happened. Generally they will be able to manually transfer the funds across.

Remember - you must check and change that Paypal Email in EVERY listing.... all new ones, and any relists or templates. It is held on a listing-by-listing basis so there is no way of globally changing it, and changes are not retrospective (don't get applied to earlier listings).