Windows phone 8.1 is terminated when phone locks

I am facing this weird issue with a windows 8.1 app I created. All the other apps I have already made stay in the app stack when the phone is locked, until I close them

This app however is closed after a minute when the phone is locked, and it is not in the app stack, I have to restart it.

It is an app that consumes a webservice, so it does not consume loads of resources when the phone is locked. When I debug it with Visual studio the app stays alive all the time. The problem occurs on the phone, not in debug mode.

I have almost no code in the app.cs, only thing I have changed is thr start startpage.

Anyone has an idea ?

Help is greatly appreciated.

Matthew