when you deploy your report do you also deploy the datasource?

In Visual Studio, if you right click on the project and select 'properties', then click on 'configuration properties | general' is the 'overwritedatasource' value (under 'deployment') set to 'true'?

If not, then any changes you make to the credentials will not be deployed. I'm no expert, but have had issues with datasources in the past.

Another thing to check is: on the properties of the datasource, general tab, click the 'edit' button adjacent to the connection string and then click the test connection button. If you are using a 'specific user name and password' these should be displayed, and if correct you should be able to connect from VS at least - then check the deploy settings as per above