A stubborn muscle group I strived to improve was back. Being tall it takes quite along time to develop alot of thickness on a tall person’s back so rather than train it twice a week originally I prioritise it to the beginning of the week after full rest of the weekend with plenty of good food so you can hit it hard.
Then after that once I had plateaued and made good gains and caught it up alot, I then split into a thickness and a width day. After rest days are a good way to bring in and improve a weak body part. I wouldn’t recommend training shoulders and legs twice a week, they get alot of work by training the other body parts, and if you can train legs twice a week then your not training them hard enough!