How to change the owner of SQL Server database tables

DBO is not the owner of the table casetable, so it cannot execute various SQL statements in the query analyzer. The prompt is:
Server: Message 208, level 16, status 1, Row 1
The object name 'casetable' is invalid.

Generally, the default owner of a table is DBO. Currently, several tables are not, so they cannot be connected using SA. How can I change it back to DBO?

Exec sp_changeobjectowner 'casetable', 'dbo'

Prompt after execution:
Server: Message 15001, level 16, status 1, process sp_changeobjectowner, row 38
The object 'casetable' does not exist or is not valid for this operation.
What's the problem? I used SA in the query analyzer to perform operations.

To include the current owner of the table, assume that the original owner is user1.

Exec sp_changeobjectowner 'user1. casetable', 'dbo'