Banana Cream Pie recipe not firming up

[Jims Pantry]

I recently tried the Prepared Pantry's Banana Cream Pie in a Vanilla Cookie Crumb Crust. The flavor was okay, but even cooking the pudding mix until very thick didn't seem to help this pie firm up like it should. Even after an overnight stay in the fridge, it breaks up when serving, and I ended up serving it with a spoon, like banana pudding. What went wrong, and how can I fix it next time?