Android webview height doesn't fill in the screen size

I am developing an android app for a videogame website and I need to make a tabbed layout. In each tab there must be a webview because I have to load on it some apposite php pages I have created.

onCreate code here

The first tab has a webview formatted in this way:

<LinearLayout xmlns:android="http://ift.tt/nIICcg"
    xmlns:tools="http://ift.tt/LrGmb4"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.leaderboardmk8.MainActivity" >

    <WebView
        android:id="@+id/webview"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

And it looks like this. The 2nd tab has this XML code:

<LinearLayout xmlns:android="http://ift.tt/nIICcg"
    xmlns:tools="http://ift.tt/LrGmb4"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.leaderboardmk8.MainActivity" >

    <WebView
        android:id="@+id/webview2"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

For some reason this webview2 looks like this: as you can see there's only a small part of the WebView component on the top. I don't know where is the problem because I typed:

android:layout_width="match_parent"
android:layout_height="match_parent"

Do you have any suggestion?