There are two items to check for the MWI View database. The tool that runs it and the database itself.
The tool is under C:\Avaya_Support\Tools\MWIView\MWIView.exe and the database is under C:\Program Files\Avaya Modular Messaging\MWI Server
When you open the tool then go to File, Open to open the database.
This tool is loaded on all MAS in the MM product but only will run where the MWI Service is running. If you only have a single MAS then you have it ready there. If multiple MAS then you will have to know where the MWI Service is running. You can find this information in the VMSC under MWI or looking at the services on each MAS.
In Exchange as well as Avaya MSS, each change to the mailbox updates the FEDB on the MAS with a 1027 event. Are you seeing these events? What event are you seeing for the extension that is not working? When you refresh the MWI on the Exchange Mailbox are you checking the Advanced & User Options button to verify that the MWI is set properly?
Check that the mailbox extension is present in the MWI View tool. If not you can do a refresh to see if it is recognized. If not, check programming on the Mailbox (Advanced & User tabs), CM programming on the station. If all ok, you may have to rebuild the MWI database for this user.