The more tag protection is for blog posts. It doesn't work on pages by default because WordPress does not allow it. You can read all about it here:
http://codex.wordpress.org/Customizing_the_Read_More#How_to_use_Read_More_in_Pages
Anyway, here is what WordPress says:
"Please remember that the "Read More" tag is used only on the Home page which shows the latest posts. It does not work in "Pages"."
with that said, if you are trying to create teaser content for your premium content then I think your best option would be to use the section protection feature of WP eMember:
http://www.tipsandtricks-hq.com/wordpress-membership/how-to-protect-a-section-of-a-post-or-page-88
You only need to protect the content that's after the more tag. So your page structure will look something similar to the following:
-----------Start of a page---------
Teaser content
<--more tag-->
[emember_protected for=2-3]
Your secret stuff
[/emember_protected]
-------------End of a page---------