Handler With IRequiresSessionState Does Not Extend Session Timeout

I have a handler, like this, using IRequiresSessionState:

public class MyHandler : IHttpHandler, IRequiresSessionState
{
// code
}

In the code, I am able to read and set Session values. I also check ensure that the caller is logged in. That all works fine.

The web site uses forms authentication, so in the web.config, I have this:

<authentication mode="Forms">
<forms loginUrl="Login.aspx" timeout="10" slidingExpiration="true"></forms>
</authentication>

The problem is that AJAX calls from the client to the server code MyHandler do not extend the life of the session!

So even if the user is busy sending and receiving data from the server, they time out 10 minutes after their last full page load.

In the handler, I've tried specifically changing a value is the session on each call, but not even that extends the time out.