The longer you have been training, the closer you will get to a point where you will find it hard to increase your Squats and Deadlifts. Mobility and technique work will definitely help, but adding some accessory work to strengthen up your weak points and give your body a different stimulus can make all the difference.
Most accessory exercises can be done after your main lifts on strength days and you should prioritise them depending on what weaknesses you have.
Here are some exercises to get you started to help improve your Squat and Deadlift:
Good Mornings – Will help strengthen you lower back, glutes and hamstrings for squatting and pulling off the ground.
DB Walking Lunges – Unilateral exercise to strengthen your legs and glutes and make sure your dominant leg is not doing all the work.
GHD Hip Extension – Glute and hamstring isolation work that will help with hip extension power.
GHD Sit Up – Intense abdominal exercises that also active glutes and hamstrings to improve dynamic core strength.
Start with these four and see if they help improve your lifting!