Real problemas with sessions and SSI...

Can someone explain to me why sessions don't work with SSI? Let me explain my case: I have a php script called miec.php, which uses sessions extensively. It's a script to show products from a catalog randomly, and in the session it keeps the products already shown, so it doesn't repeat... it works just fine. So... if I go to my browser and point to miec.php, and keep reloading it, I can see it working... the sessions get updated perfectly and then reseted when the maxinum number of allowed products is reached... Then I have another file called test.shtml which includes miec.php via SSI (<!--#include virtual="miec.php"-->), the include is perfect... even the cookies holding the userId works... but the session simply doesn't work... When I load test.shtml, a session is create by miec.php, I can even see the file on the server. But if I reload the page, another session is created instead of using the same, so it's not persistent... thus, it doesn't work at all. The weird thing is: if I call miec.php directly, the session gets created normally, if then I point the browser to test.shtml, the session persists normally, and then everything works as expected... I believe this is not the expected behaviour... Does sessions work with SSI or not?