Can't debug fragments on android studio

I'm using Android-Studio and since last week I couldn't stop at any break point set to any fragment. If I put a break point in the activity, the debugger breaks as expected. If the debugger comes to a line like: MyFragment f = new MyFragment(); or any line with a reference to a fragment, and I press "Step into" the debuggeperforms the actions but I still see the debugger stands on the lineMyFragment f = new MyFragment();`

Moreover, I noticed that I get a messages like this in the Debug section:

fragment = No such static field: 'fragment'
chgPicBtn = No such instance field: 'chgPicBtn'

I don't know if it's related but it was worth mentioning.

I've been looking for similar problem online and found nothing. Thanks for the help

Edit: This is a function that calls the activity:

public void displayView(int position) {

    String tag = "";
    fragment = null;
    switch (position) {
        case 0:
            fragment = new NewFragmentMain();
            if (chgPicBtn != null)
                chgPicBtn.setVisibility(View.GONE);
            tag = "NewFragmentMain";
            break;
        case 1:
            fragment = EditProfileFragment.newInstance(!bool);
            tag = "EditProfileFragment";
            if (chgPicBtn != null)
                chgPicBtn.setVisibility(View.VISIBLE);
            break;
        ...
        default:
            break;
    }

    if (fragment != null) {
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.replace(R.id.container_body, fragment, tag);
        fragmentTransaction.addToBackStack(tag).commit();
    }
}

The constructor of NewFragmentMain is empty.

EDIT:

The problem was due to:

debug {
        minifyEnabled true
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }

in the gradle file. I turned it to false and everything worked.