It's not just just study that's difficult.

I have a bit of a question. I see study used instead of studying too often text books. I feel like this is just grammatically incorrect. i will give a few examples.

You need to focus on your study.
Not just study is difficult.
You need a better balance between study and your social life.
It's not just study that's difficult.

I see this used too often in text books that I use. How can I convince people that it just doesn't sound right to use study. I feel like studying sounds better in this context.