The  syntax of the hint is incorrect.

All the tables that are used in the query are  not analyzed.

There might be a  conflict with another hint.

Some  hints requires a change inParamater values in init.ORA to be set for it to work.

The table name was aliased in the query, but you used the table name, not the alias,
in the hint.

Some hints are Oracle version Specific. You may be using a hint in your database which is meant for higher version of  Oracle rather than the Oracle database you are using.

You don’t understand the correct application for the hint.

There is a software bug .

Hint is just a instruction to Oracle optimizer , But oracle can ignore your hint Since it finds another best possible way to execute a query.