>What might I be doing wrong

Seems you added ,more starch...in the form of cornstarch.

If you only added more fruit into the apple pie filling you should not
add starch anymore.

Take note the apple pie filling is already stabilized with waxy maize
starch and adding more will absorb more liquid making your apple
filling dry textured.

On the other hand just as Mary suggested try the recipe from the
scratch using any known cookbook.