Google Analytics can't track orders accurately!

I have installed google analytics code into my zen-cart template. So it can track all pages of my site.
I find that it can't track transactions correctly.
I have set the conversion goal page as checkout_cuccess page.
Goal URL: ^/index.php?main_page=checkout_success

But it can only track less than half of all my orders.
I don't know what the problem is.

Is it because some clients don't go to checkout success page with paypal IPN method?

Also I am using google optimizer too. I know doubt the accuracy of google optimizer data.

I searched and found 4 addons about conversion tracking.

1. Simple Google Analytics
2. Generic Conversion Tracking
3. Supertracker
4. User Tracking Interface [UTI]

http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=412
http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=1235
http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=683
http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=1374

I want to know more detailed information of each buyer/visitor. Like where they are from, what pages they read, and so on.

I am using google analytics now, but not installed 'simple google analytics' addon.
I am not sure if it's necessary to install 1 or 2 of the 4 addons above.

Please advise, thanks

The best tracking is Google Analytics by Andrew. Very easy installation and has the advanced E-Commerce features as well. Highly recommended. I havent tried the others you mentioned, but i think google includes a lot of features that might cover all of the above.

The only drawback is that it only tracks when the user has Javascript enabled.