Link to login doesn't work when selected from the menu

I'm mystified by this one. I have a menu on my site that includes a login. The link is:

href"**https://www.edufone.com/store/index.php?main_page=login">**REGISTER / LOGIN</a></td>

If I copy and paste that link to my browser, the login page displays.

However, if I click on that link from any page on my site that is not part of the zen-cart store (such as from the Home Page, from "FAQ", etc., nothing happens.

However, if I first click on "Products" and then "Login/Register" which is also linked to this same page, it displays the login page. The link in the store/includes/templates/myTemplate/common/html_header is identical:

I've also tried changing the link to http instead of https but it makes no difference. Any assistance would be greatly appreciated.

jdl