If someone forgot the Dhuhā prayer or was busied from it, do they make up for it?
Yes, they make up for it if they are consistant upon it due to the generality of the Hadith;
من نام عن صلاة أو نسيها فليصلها حين يذكرها
“Whoever missed a prayer due to sleep or forgetfulness, then let him pray it when he remembers it.”
Shaykh Abu Hamza Hassan bin Muhammed Bā Shu’ayb -may Allah preserve him-
Abu ‘Abdillāh ‘Omar bin Yayha Al-‘Akawi