Potentially 2 things.
The current configuration has different cookie path/domain, so your user session is never maintained - which results into an empty cart even right after adding an item.
You can check/verify this by using Chrome DevTools, navigating to "Application" tab and then Cookies in the left menu. Find one that has the name "frontend" and verify that the "Domain" and "Path" columns for that are correct.
If not - this can be fixed in System configuration: Web -> Session Cookie Management-> Cookie Path / Cookie Domain or by making edits directly in the database within core_config_data table.
It is likely that your 1.7 has had patches installed which makes it so form_keys are required for forms to accept your post data.
To verify this go to the product page in front-end and check the page source and verify that you have a form_key input within your add to cart form. The input field should look like this:.
<input name="form_key" type="hidden" value="......." />
If you can not find this make sure you add it into your add to cart form via XML layout or directly into phtml by adding:
<input name="form_key" type="hidden" value="<?php echo Mage::getSingleton('core/session')->getFormKey() ?>" />