The pie may not have baked long enough. Cornstarch has to reach a certain temperature to activate and gel, so if a pie isn’t baked long enough, the cornstarch may not set properly. While a golden crust can often be one indicator of doneness, you should also look for signs in the filling. The fruit filling should be bubbling around the edges and at the center of the pie. For the most fool-proof check, insert an instant-read thermometer into the center of the pie filling– it should reach 200°F-205°F before being removed from the oven.   The strawberry rhubarb pie filling will also be quite runny straight out of the oven, so it’s important to wait for the pie to cool completely before you slice it. While it cools, the filling will firm up nicely and become soft and jammy.