While building muscles, take more proteins for muscles to grow, ideally intake is 1gm of protein per pound of body weight.
You need to eat more, without enough calories, body can enter catabolic state, destroying muscles you worked so hard to gain.
Rather than focusing on one muscle group, target alternate muscle groups to reduce risk of injury and rebuilding of muscles.
Sleep is very important for muscle recovery & growth, sleep for 7-8 hours per night to give enough time for muscles to grow.
Focus on compound exercises like squats, push-ups, deadlifts that works on more than one muscles to build more strength.
Slow down your rep speed and concentrate on form, muscles grow more when they are under tension for longer periods of time.