getItemCount() of my RecyclerView adapter returns 0 even though I have items in it

My RecyclerView returns a getItemCount() of 0 even though I have items in it. What can be the reason?

In Activity

int length = mAdapter.getItemCount();

In Adapter

@Override
public int getItemCount() {
    return reportlist.size();
}

onCreate() method in my activity

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_report_listing);

mRecyclerView = findViewById(R.id.recyclerView);
flightViewModel = new ViewModelProvider(this).get(FlightViewModel.class);
userViewModel = new ViewModelProvider(this).get(UserViewModel.class);

mLayoutManager = new LinearLayoutManager(ReportListingActivity.this);
mAdapter = new ReportAdapter(reportitems, ReportListingActivity.this);

mRecyclerView.setLayoutManager(mLayoutManager);
mRecyclerView.setAdapter(mAdapter);

userViewModel.getAllUsers().observe(ReportListingActivity.this, new Observer<List<User>>() {
            @Override
            public void onChanged(List<User> users) {
                user_details = getUsers();
                user_name = user_details[0];
                user_id = user_details[1];

                flights = flightViewModel.getFlightReports(user_name,user_id);
                if (flights.size() == 0) return;

                reportitems.clear();
                reportItemsCopy.clear();

                for(int i = 0; i <flights.size();i++){
                    String flightnumber = flights.get(i).getFlightNumber();
                    String departuredate = flights.get(i).getDate();
                    String status = flights.get(i).getFlight_closure_status();
                    String user_name = flights.get(i).getName();
                    String user_id = flights.get(i).getStaffID();


                    reportitems.add(new ReportItem(flightnumber,departuredate,status, user_name, user_id));


                }
                mAdapter.notifyDataSetChanged();

            }
        });
}