Class inheritance question in magento (google analytics)

Question

In a custom module I'm inserting universal analytics so I have to change a few functions from Ga.php file

In Ga.php there is:

class Mage_GoogleAnalytics_Block_Ga extends Mage_Core_Block_Template
{
//google analytics functions
}

If I do like this:

class Custom_Module_Block_Universalanalytics extends Mage_GoogleAnalytics_Block_Ga{ 
//a couple of functions that I change from Ga.php
}

it doesn't work.

But if I do like this:

class Custom_Module_Block_Universalanalytics extends Mage_Core_Block_Template{  
//all functions from Ga.php plus the ones that I change
}

it does work.

So my question is why wouldn't it work if I extend the Mage_GoogleAnalytics_Block_Ga class which already has all the functions?

Update: Here is my config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Custom_Module>
            <version>0.1.0</version>
        </Custom_Module>
    </modules>
    <global>
        <blocks>
            <universalanalytics>
                <class>Custom_Module_Block</class>
            </universalanalytics>
        </blocks>
        <helpers>
            <universalanalytics>
                <class>Custom_Module_Helper</class>
            </universalanalytics>
        </helpers>
        <models>
            <universalanalytics>
                <class>Custom_Module_Model</class>
            </universalanalytics>
        </models>
    </global>
    <frontend>
        <events>
            <checkout_onepage_controller_success_action>
                <observers>
                    <universalanalytics_order_success>
                        <class>universalanalytics/observer</class>
                        <method>setUniversalanalyticsOnOrderSuccessPageView</method>
                    </universalanalytics_order_success>
                </observers>
            </checkout_onepage_controller_success_action>
            <checkout_multishipping_controller_success_action>
                <observers>
                    <universalanalytics_order_success>
                        <class>universalanalytics/observer</class>
                        <method>setUniversalanalyticsOnOrderSuccessPageView</method>
                    </universalanalytics_order_success>
                </observers>
            </checkout_multishipping_controller_success_action>
        </events>
        <layout>
            <updates>
                <universalanalytics>
                    <file>universalanalytics.xml</file>
                </universalanalytics>
            </updates>
        </layout>
    </frontend>
</config>

Thanks!

Update: Folowing the comments, indeedthe issue is with this function:

protected function _toHtml(){
    if (!Mage::helper('universalanalytics')->isUniversalanalyticsAvailable()) {
        return '';
    }
    return parent::_toHtml();
}

Mage::helper('universalanalytics')->isUniversalanalyticsAvailable() is returning true, but, as I read here https://stackoverflow.com/questions/11179732/overwritten-block-appears-in-addition-to-module-overwrite return parent::_toHtml(); will return the google classic analytics object.

Answer 1

In the class that you extend, there is a _toHtml function that will return an empty string is isGoogleAnalyticsAvailable() returns false. And since your class extends this one, it inherits this function (unless you rewrite it)