Hello I have created six buttons in an app in android studio inside the Relative Layout. The buttons show OK in the preview mode of android studio but they second last one does not show when I install my app to test on a phone or an emulator. What could be the problem
Here is my code

<?xml version="1.0" encoding="utf-8"?>
    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:paddingBottom="@dimen/activity_vertical_margin"
        android:paddingLeft="@dimen/activity_horizontal_margin"
        android:paddingRight="@dimen/activity_horizontal_margin"
        android:paddingTop="@dimen/activity_vertical_margin"
        tools:context="com.example.nelson.kuzaapp.MainActivity">

        <ImageView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:src="@drawable/logoo"
            android:contentDescription="@string/logo"
            android:id="@+id/logo"
            android:layout_marginTop="-500dp"
            />
       <!--Make query button-->
        <Button
            style="?android:textAppearanceSmall"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="150dp"
            android:text="@string/make_farming_query"
            android:id="@+id/buttonAskExpert"
            android:layout_below="@+id/logo"
            android:layout_alignParentTop="true"
            android:layout_alignParentRight="true"
            android:layout_alignParentEnd="true"
            />
        <!--View products Button-->
        <Button
            style="?android:textAppearanceSmall"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="16dp"
            android:text="@string/view_products"
            android:id="@+id/buttonViewProducts"
            android:layout_below="@+id/buttonAskExpert"
            android:layout_alignLeft="@+id/buttonAskExpert"
            android:layout_alignStart="@+id/buttonAskExpert"
            />
<!--Farmer Login button-->
        <Button
            style="?android:textAppearanceSmall"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="16dp"
            android:text="@string/farmer_login"
            android:id="@+id/buttonFarmerLogin"
            android:layout_below="@+id/buttonViewProducts"
            android:layout_alignLeft="@+id/buttonViewProducts"
            android:layout_alignStart="@+id/buttonViewProducts" />
<!--Farmer registration-->
        <Button
            style="?android:textAppearanceSmall"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="16dp"
            android:text="@string/farmer_registration"
            android:id="@+id/buttonRegisterFarmer"
            android:layout_below="@+id/buttonFarmerLogin"
            android:layout_alignLeft="@+id/buttonFarmerLogin"
            android:layout_alignStart="@+id/buttonFarmerLogin" />
<!-- Twitter button-->
        <Button
            style="?android:textAppearanceSmall"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="16dp"
            android:text="@string/follow_us_on_twitter"
            android:id="@+id/buttonFollowTwitter"
            android:layout_below="@+id/buttonRegisterFarmer"
            android:layout_alignLeft="@+id/buttonRegisterFarmer"
            android:layout_alignStart="@+id/buttonRegisterFarmer" />
<!--Facebook button-->
        <Button
            style="?android:textAppearanceSmall"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="16dp"
            android:text="@string/like_us_on_facebook"
            android:id="@+id/buttonLikeFacebook"
            android:layout_below="@+id/buttonFollowTwitter"
            android:layout_alignLeft="@+id/buttonFollowTwitter"
            android:layout_alignStart="@+id/buttonFollowTwitter" />


    </RelativeLayout>