The shortening was cut in too much. Cut in shortening so it resembles coarse crumbs, not any smaller.
The pastry was overhandled.
Too much water was used. Stir in only enough water to moisten the flour so a ball forms.
Next time, bake crust longer or at a higher temperature.
Before adding the filling, brush the crust with a beaten egg white, then bake.
Add the filling to the crust right before baking.
Make sure the crust is rolled out evenly.
Your oven may bake unevenly. Next time, rotate the pie during baking.
Too much filling was used. Reduce the amount of filling or use a deep-dish pie plate.
Keep the filling warm while making the meringue. Then spread the meringue over hot filling
Be sure to seal the meringue to the edge of the pastry before baking.
Meringue was baked at too high a temperature or was underbaked.
For best results, meringue should not be made on a humid or rainy day.
Beat the filling only until combined.