Chances are if you followed the recipe and it still seems runny you just need to let the raspberries simmer a little longer on the stove. But also bear in mind that it will continue to thicken more as it cools. You can usually fix this by putting the filling back in the pan and cooking it a little longer (even if you previously refrigerated it). Also, if you are using fresh raspberries instead of frozen, they sometimes don't thicken quite as easily so you may need to add a little extra cornstarch.