Does the number you want blocked appear in the Block List view of the app? If it does not, you will need to add it by manually entering it in.
In some cases, it is necessary to tap the 'Check for Updates' button located in the About view of the CallWatch app to ensure that you have the latest version and all app settings are correct.
NOTE: Please make sure that any security apps you may have installed are set to exclude CallWatch. Examples of those types of apps are firewalls, task killer, anti-virus apps, and any app that is attempting to perform an action when calls come in.