Can't change MAIL_DRIVER to mailgun in Laravel 5.4

I updated MAIL_DRIVER in .env file of production server. I run these commands also:

php artisan config:clear
php artisan config:cache

However, they did not change MAIL_DRIVER. Server continues to send mail via old MAIL_DRIVER. I got from controller env("MAIL_DRIVER"), and it gets nothing (NULL).

How can I solve this problem.

via Chebli Mohamed