Android algunos de los elementos de vista no visibles

Programas de LinearLayout antes de CheckBox tiene su altura establecida en MATCH_PARENT(and it fills all the parent's height) and the parent LinearLayout of both has the orientation set to vertical so the CheckBox es expulsado de la pantalla.

Establecer la altura del LinearLayout que contiene el SeekBar a WRAP_CONTENT por ejemplo.

You need to set your inner Linear Layout's height and width to wrap content. Try this.

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal" >

        <SeekBar
            android:id="@+id/seek"
            android:layout_width="300dip"
            android:layout_height="wrap_content"
            android:progress="50" />
    </LinearLayout>

    <CheckBox
        android:id="@+id/CheckBox"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="somestring" />

</LinearLayout>

Prueba esto :

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical"
android:weightSum="10" >

<LinearLayout
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:layout_weight="9"
    android:orientation="horizontal" >

    <SeekBar
        android:id="@+id/seek"
        android:layout_width="300dip"
        android:layout_height="wrap_content"
        android:progress="50" />
</LinearLayout>

<CheckBox
    android:id="@+id/CheckBox"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_weight="1"
    android:text="somestring" />

</LinearLayout>

In your layout chose one element that you want to be flexible in size, say height wise. Then make its height="0dp" y weight="1". Make heights of rest of elements: height="wrap_content".

Also you may have given discreet size of some dp to your other elements, and hence their total height runs out of available screen space OR there are too many elements that they go beyond your screen height. In this case, wrap your layout in a ScrollView.

change your LinerLayout definition it cannot be android:layout_height="match_parent"

<LinearLayout android:orientation="horizontal" 
    android:layout_width="match_parent"
    android:layout_height="0px"
    android:layout_weight="1">

if you set match_parent for linearlayout it consumes all content so wrap_content is better. But if you set it as I wrote checkbox will be at bottom page a linearlayout will consumes remaing part of screen.