Revoke SQL doesn't take effect

I have two postgresql accounts created by someone else who I do not know (So I do not know setting for those accounts and tables created). One is read only account e.g. read_only_user (This can perform select operations only). The other is admin account e.g. admin_user (This can perform grant, revoke, CRUD, and so on operations).

The read only account can query (select sql) a table (suppose it's called table1) under a specific schema (suppose it's schema1). For instance select * from schema1.table1. Now I received a request to revoke select for that read only account on table1. So I execute

revoke select on schema1.table1 from read_only_user

psql returns REVOKE string (or something similar showing the sql execution was successful) on console. However, when check with read_only_user account. I am still able to query table1. Searching the internet, [1] looks like the closest to my problem. But I do not find solution in that thread.

So my question:
What steps do I need to perform in order to exactly revoke select from read only user account for a particular table? So the read only user account wont' be able query that specific table with select permission revoke (psql should returns info like permission denied).

Thanks

[1]. https://www.postgresql.org/message-id/00ce01cbef04%24910606b0%24b3121410%24%40yahoo.com