Your lemon bars are likely runny because the lemon curd hasn't fully cooked. Either you didn't thicken the custard adequately before pouring it onto the shortbread crust, or it didn't bake long enough in the oven. To prevent runny squares, make sure the curd is thick enough to coat the back of your spoon or has reached a temperature of 175 °F (79°C) before pouring it into the base and remove the bars from the oven only once the filling is firm, yet jiggly.