LocationManager limitation on samsung device

Hello everybody, this is my situation:
I developed an app that has a widget. Tapping on the widget (and thus having the app out of focus in the background) I call a Service or an IntentService to have some GPS-related data (the position). I tried with getLastKnownLocation and requestSingleUpdate. Both works but after a dozen of seconds that the app is in the background they stop working.
The example is, when using getLastKnownLocationI get a value and after few seconds, the same service, return null.

Is there any limitation of the usage of those methods? Can you explain to me how to solve this problem?