Has anything changed with your routine? I would keep the door closed with you outside with her to make sure she went outside. When she does go, make a big deal, praise her, offer her a small treat. But try keeping her meals at regular times in the day. My dogs get fed twice a day. At 7am and at 5pm.
But they are big dogs, so maybe yours should be fed 3 times a day?
All the best, and be patient, it'll happen.