getDownloadURL and post to Firebase Database on Android sometimes doesn't work

Question

I am trying to upload an image to Firebase when creating a new user and get the image downloadUrl and post it to my realtime database as part of the users details. Sometimes the downloadUrl gets posted, sometimes it doesn't get posted but the image always uploads successfully. This is my code, am I doing something wrong? Please help...

upload image method

private void postImage() {
        String path = "userProfiles/" + UUID.randomUUID() + ".jpeg";
        StorageReference userProfilesRef = storage.getReference(path);
        userProfilesRef.putFile(filePath).addOnSuccessListener(new OnSuccessListener<UploadTask.TaskSnapshot>() {
            @Override
            public void onSuccess(UploadTask.TaskSnapshot taskSnapshot) {
                 downloadUrl = taskSnapshot.getDownloadUrl().toString();


                //if the upload is successfull
                //hiding the progress dialog

            }
        });
    }

registering user method

private void registerUser(){
        postImage();
        //getting email and password from edit texts
        String email = editTextEmail.getText().toString().trim();
        final String username = editTextUsername.getText().toString().trim();
        final String password  = editTextPassword.getText().toString().trim();



        //checking if email and passwords are empty
        if(TextUtils.isEmpty(email)){
            Toast.makeText(this,"Please enter email",Toast.LENGTH_LONG).show();
            return;
        }

        if(TextUtils.isEmpty(password)){
            Toast.makeText(this,"Please enter password",Toast.LENGTH_LONG).show();
            return;
        }



        //if the email and password are not empty
        //displaying a progress dialog

        progressDialog.setMessage("Registering Please Wait...");
        progressDialog.show();

        //creating a new user
        firebaseAuth.createUserWithEmailAndPassword(email, password)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {

                        //checking if success
                        if(task.isSuccessful()){

                            String user_id = firebaseAuth.getCurrentUser().getUid();
                            UserInformation userInformation = new UserInformation(username, halls, levels, downloadUrl);


                            databaseReference.child(sexs).child(user_id).setValue(userInformation);


                            Toast.makeText(MainActivity.this,"Successfully registered",Toast.LENGTH_LONG).show();

                            //finish();
                            //startActivity(new Intent(getApplicationContext(), Users.class));
                        }else{
                            //display some message here
                            Toast.makeText(MainActivity.this,"Registration Error",Toast.LENGTH_LONG).show();
                        }
                        progressDialog.dismiss();
                    }
                });

    }