[Q] I Rooted Device, But I Can't Edit System Files?

Hi friends;

I have HTC M8 Marshmallow with Sense 7.
I rooted my device with SuperSU 2.65.

But;
I can't edit build.prob file in system folder.
I can't put apk and lib files to system->priv-app folder.

In fact ;
I edited build.prob file in system folder. (I used Root Explorer)
I put apk and lib files to system->priv-app folder.

But, after I rebooted device edited build.prob file returned original and
deleted apk and lib file that I put to system->priv-app folder.:(

Before these applications I mounted system folder R/W

Why did this happen?
How can i solve this?:(

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