Android layout weight doesn't work

I got this layout:

<RelativeLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginBottom="6sp"
    android:layout_marginLeft="16sp"
    android:layout_marginRight="6sp"
    android:layout_marginTop="6sp" >

    <TextView android:id="@+android:id/title"
        android:layout_width="wrap_content" 
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceListItem"
        android:inputType="none" />

    <TextView android:id="@+android:id/summary"
        android:layout_height="wrap_content" 
        android:textAppearance="?android:attr/textAppearanceSmall"
        android:layout_below="@android:id/title" 
        android:maxLines="2"
        android:layout_width="wrap_content" />

</RelativeLayout>

<!-- Preference should place its actual preference widget here. -->
<!-- android:id="@android:id/widget_frame" -->

<LinearLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentRight="true"
    android:layout_marginBottom="6sp"
    android:layout_marginLeft="6sp"
    android:layout_marginRight="6sp"
    android:layout_marginTop="10sp"
    android:gravity="center_vertical"
    android:orientation="horizontal"
    android:weightSum="1" >

    <SeekBar
        android:id="@+id/change_seekbar"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_weight="0.8"
        android:paddingLeft="20dip"
        android:paddingRight="20dip" >

    </SeekBar>

    <TextView
        android:id="@+id/Text_Box_seek"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="5dip"
        android:textSize="16dip" >
    </TextView>
</LinearLayout>

I want the seek bar to take 80% of the width in the inner linear layout, but the seek bar stays very small.. why won't it take the rest of the space?

Change the layout width of your LinearLayout to match_parent or set a specific value for it.

Explain If you use LinearLayout width = wrap_content, the width of LinearLayout will depend of the childs.
And you also set the weight for the childs, so the the width of the child will depend on the parent.
=> (width of parent depend on childs, width of childs depend on parent) then I think the View will don't know what to do