A number of things are possible:
You haven't correctly provided IDs to specify the Org Unit (course), the particular grade object (line item in the gradebook), or user
The calling user making this API call does not have permission to use this call to see the grade value
For example, if your calling user context is a student user (i.e. the person to whom the grade is assigned), then it's likely that this user will not have permission to issue the call you describe -- typically, the call you refer to is intended for use by administrative users.
When you want a calling user to retrieve "their own" grades, you should use the 'my grades' family of calls instead. In particular you might want the call to fetch back "my grade" for a particular grade object here.
Additionally, note that the APIs to fetch back final grade values are separate from the calls to get grade values for other grade objects, and also the final calculated grade value isn't accessible through the API, only the final adjusted grade value (and even then, only when the final grade is "released").