In making a 'crumb' (gingersnap, graham, vanilla wafer, etc.) crust/base for a dessert or bar - why do some turn out hard and almost impossible to cut - and others turn out nicely?

The one I just made (13 x 9 pan) called for 2 cups of the crumbs, 1 stick butter, 3 tablespoons sugar - and lemon or orange peel. I pressed it in the pan - and baked 6 to 8 minutes at 325.

Removed - and upped the oven to 375 - when my filling/topping was ready I spread/spooned it on - and baked it 40 - 42 minutes until it was done.

Cooled. Cut into squares and tho it was tasty - the bottom crust did not want to stay with the filling.

Should I NOT have baked it first? Should it have been a thicker base - like 3 cups crumbs, etc.?

The latter temp and baking time was based on the type of filling/topping I had . I could probably lower it to 350.

Any ideas? It's not the type of crumb base I want - but am looking for guidelines before I try this version again. Thanks for any help/ideas in advance.