OptinLinks keeps track of who accesses a particular secure page.
If you've already accessed the page and completed the opt in form, it
will not present you with the form again. If you would like to
re-enable the form for testing purposes you can use one of the anonymous
browser capabilities like Incognito mode in Chrome or InPrivate
Browsing in IE.
Each new session using the browsers in this anonymous mode will reset your profile for the plugin and will cause the form to re-appear.
OptinLinks also tracks who has signed up through a form and if you've already been sent an email, will not be sent another one. There are a number of temporary address services available that would allow you to use a different email address with each request such as Mailinator.
Things to check...
Make sure that the full URL in the browser matches the URL on the secure page exactly - down to https:// vs. http:// and trailing slashes on the URL.