If you ever get to a situation where you don't see all the emails in a particular folder, you can fix the situation by dropping the local cache that will resynchronize the content from the server.
Follow the steps below to achieve that.
1. Right-click on a folder and select Properties
2. Under Properties select Repair and press Repair
You should shortly see the missing emails from the server.
Use the Repair button with caution, as it deletes the whole local cache for a particular folder, including items that were not uploaded to the server yet.