Hi,
There could be a number of reasons but you will have to test various settings.
1. Can you please make sure the plugin is enabled? i.e. under the General settings tab make sure the Enabled option is checked.
2. Sometimes the notice may not be displayed due to a cache issue. Can you confirm if you are using any cache plugin? In that case can you try and enable the AJAX options under the General settings tab.
3. Sometimes it might have to do with the position of the message, if it is in the automatically refreshed cart area or out of that area. In that case can you try and change the Cart position from the Cart tab.
Also good to make sure you have enabled the Cart notice or relevant notice section where you want the message displayed.