Membership not protecting content

Membership has situations where it seems to not protect content. I have two access levels, visitor and member.

Protecting content in the /trade-ideas/ category by adding the category to negative for visitors, and also have added /trade-ideas/ to a URL group and added it as a negative item for visitors.

When a non-logged in visitor goes to the site and clicks for an article that is protected, the first time it goes to the /protected/ page. But if they hit back, then try the same article again, they get the article instead of the /protected/ page.

For example, see http://www.optionsideacentral.com/tag/earnings/ pick any article, click, get /protected/ page. Go back, click same article again, it comes up.

Obviously this is a big deal, the protection doesn't mean much if the user only has to hit back and try a second time to get in.

Appreciate your help in advance.