In addition to what Jock said...
Cake flour is from soft wheat, but then chlorinated to further break down the strength of the gluten. You will see that it is much whiter than say unbleached bread flour. You will often see chlorine and benzoyl peroxide as ingredients.
Pastry flour is also from soft wheat like cake flour, but it is off-white not true white/bleached. It is actually somewhat similar to all purpose flour in terms of gluten.
So cake flour has less gluten-effect as the gluten is more broken down.
Most consumer product "cake and pastry" generally speaking, provides a versatile product with less gluten than AP flour, to improve the success a home cook would have with cake, and pastry. For pastry, if a less experienced baker's techniques are bringing out too much gluten by other physical elements, such as not cold enough ingredients, overmoistening or overworking the dough, a too warm fat too mushed in with the flour, etc, it provides a little protection against that that if there's less gluten. So "cake and pastry" will help counter bad technique by cutting the gluten in the flour.
So back to your original question, it sounds as though your dough is coming out too glutinous being difficult to roll out. You could check the coldness of your flour, fat, ice water, etc., check the technique or amount of fat and water, and/or if it was working great with a commercial "cake and pastry" and you wanted to similate that, then I would say hesitantly that you could try to "up" the CAKE flour in that blend, as it may have been likely that having a lower gluten content in your previous use of "cake and pastry" was helping you by covering up other issues in pastry technique.
Now I would say there is a trade off, that bleached out cake flour doesn't have as good taste as a less processed pastry flour, so if you can get the method improved for coldness etc so too much gluten isn't brought out of the flour, you will get a better taste and pastry than just adding lots of cake flour. Can you work in a cool space instead of a hot kitchen, you could even freeze your shortening, make sure your flour is cool, etc., to help keep the gluten from coming out of the flour. The key is to get the flour and fat crumbed in to each other without the fat "soaking in" to the flour. Then adding barely enough ice water to barely make it come together and not overmixing. (Your recipe might have a little too much water, and the cake flour was helping counter that).
So I would say those are your options, you could try 50/50 to try to simulate what you had before, or try to improve the pastry technique to get a good product with more pastry flour. If you want a better pie crust, the latter is a much better option.
One should be able to make a good pie crust even from bread flour (which is higher gluten than AP), but the technique has to be right, cold ingredients, fat still in tiny particles, no overmixing, etc. I normally use organic unbleached all purpose flour because that's what I keep around in quantity. Personally I would never use cake flour in pie crust, but you could if that's how you want to go. I know top pastry chefs who use bread flour.
Hope this explains the root of the problem.