Will someone who forgets to recite the morning and evening adhkar one day become unprotected from the devil's schemes on that day?

There is no doubt that whoever adheres to the remembrances (dhikr) of the morning and evening will be protected by Allah from all harm. This promise has been given in the Hadith: "No servant says, every morning and every evening: 'In the Name of Allah, with Whose Name nothing on earth or in the heavens can cause harm, and He is the All-Hearing, the All-Knowing,' three times, except that nothing will harm him." Narrated by At-Tirmidhi. As for one who does not recite these remembrances, this protection does not include him, and his matter remains subject to the will of Allah.