EventBus : Activity does not receive event when app is in the background

I'm using EventBus to communicate between Activity and Service. Today I got a problem and don't know why.

1. I have Activity, Fragment and Service. All of them are working fine.

2. In Activity and Fragment I registered them to Receive events which delivered from Service

3. In Activity and Fragment, I un-register them when onDestroy() was called.

4. In normal cases, when Services delivers events, Fragment and Activity can receive those events and work well.

5. But when App is pushed on the background (by presses Home or Power button), only Fragment receives events which delivered from Service, and Activity does not receive them.

6. I did not do anything in onPause() both of Activity and Fragment.

Question:

Is there any explanation for that? And how can I make my Activity receives event like Fragment did when app is pushed on background?