The Network Performance Monitor in the Engineer's Toolset is designed for on the spot ad hoc monitoring of devices. Once the application is closed, the application will stop monitoring. There is another limitation where the database is only able to store a very small amount of data at 2GB.
If you are looking for a full 24x7 monitoring, you will want to look at Network Performance Monitor which is designed for long term data collection of 1 year or longer, and will run as a service on a server that you can remotely connect to via a Web Console. NPM will poll a lot more data than the Toolset's Performance Monitor including Routing details, Network Topology, Alerting, and Network Topology on top of the application that you are using.