Session info getting lost with ajax requests

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I have one of those annoying problems I'm hoping someone can help me with.

We've implemented a fair amount of ajax type functionality on our site and for about 1% of our users whenever I try to access concrete5 session set information in the php file that is called by the ajax request, the session information isn't set causing problems.

For example, if I call u->isLoggedIn it will return false even though when the navigate the site it's fine (it's only when I call something in the background via an ajax request).

There's nothing consistent in terms of people who report this problem (sometimes mac, sometimes windows, sometimes IE, sometimes safari etc.).

Has any one had a problem like this? Sometimes I ask the person to try again and it works fine the next time.