Muscle gains. All help and advice welcome

Sooo ive reached this stage before aswell .. and ive got your answer.
Your muscles have become stronger with all the weight training you have been doing. This means that although you are doing some pretty intense workouts the muscle itself is too healthy for the workout to have much impact. Remember growth is only a result of muscle tearing and it's now become too strong for your workout.
You now need to change your workouts but it's a good thing. Each night it's best to do a single muscle group, target with different workouts, do drop sets, supersets, literally destroy it. The next night move to the next group etc.
Also note I wouldn't recommend the above unless you were at this stage now.
I hope this helps. Good luck