checkbox in custom ListView and selectAll button bug

I am facing a problem now that I really don't know what is going on.

First let me explain what I want to do. I am having a custom ListView that show my user detail and a checkbox beside it. At first, if I checked all the checkbox, the selectAll checkbox will not check on by itself.

Therefore, I have implemented the codes below. It works perfectly at first, but in the end I found out that if I have too much item that require me to scroll down, the system like detected that I have uncheck the checkbox and help me to deduct my "count".

@Override
public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
    if(buttonView == selectAll){
        selectAll.setChecked(isChecked);
        for(int i = 0; i < theList.size(); i++){
            theList.get(i).setSelected(isChecked);
            theList.get(i).setCheckAll(isChecked);
        }
        notifyDataSetChanged();

    }else{
        int position2 = (Integer) buttonView.getTag();
        if(isChecked){
            theList.get(position2).setSelected(true);
            System.out.println("count here positions:"+ position2);
            count++;
            System.out.println("count here:"+ count);
            if(count == theList.size())
            {
                selectAll.setChecked(true);
            }
            //String this_is_the_comment = theList.get(position2).getApprComments();
            //System.out.println(this_is_the_comment);
        }else{
            theList.get(position2).setSelected(false);
            count--; //when i scroll down the if condition will come to here
            System.out.println("count here: -- "+count);
            if (selectAll.isChecked()) {
                selectAll.setChecked(false);
                for (int i = 0; i < theList.size(); i++) {
                    theList.get(i).setSelected(true);
                    theList.get(position2).setSelected(false);
                }
            }
        }
    }

Please kindly help me with this bug. I am lost and stuck for 3 days.