Have you ever driven your car in 1st gear without shifting? You push the gas harder and harder, but at a certain point, you don’t continue to speed up. You just burn a ton of gas (energy).
Have you ever pedaled a bike down hill in 1st gear? You can pedal as hard as you want, but the bike is moving fast enough that you have little effect.
No matter what we are doing in life, if we are not doing it at the appropriate intensity, it will have little effect!
A common consult I see in with shoulders is the stiff shoulder. The therapist has been mobilizing and saw initial gains, but the gains did not continue. The first thing that comes to my mind when I hear this is, “Did they shift to second (or beyond) gear?”
A joint mobilization influences neural pathways as well as mobility of the capsule. While a mobilization of Grade IV likely influences the neural pathways at any position, if the joint capsule is only minimally hypomobile in the position you are mobilizing, then you may only have a minimal effect.
Let’s put this into action and think about a patient case. Your patient is lacking shoulder internal rotation and you made initial gains with posterior joint mobilization in 90 degrees of abduction. Now there has been a plateau. What can you do? One thing you can consider is progressing the tension of the posterior capsule. You could accomplish this by placing the shoulder in either internal rotation or increasing to neutral horizontal adduction. I most often choose neutral horizontal adduction. It is a comfortable position for me as well as for the patient. 
I do have a caution when you try this with a patient. Make sure to ask the patient where they are feeling the mobilization. If they state they are feeling it anteriorly then they are experiencing an impingement-type symptom and you need to reduce the amount of horizontal adduction or work on anterior soft tissue prior to proceeding.
You can use this with any joint in the body. Need to increase ankle dorsiflexion? Dorsiflex the ankle and then perform talocrural posterior mobilizations. 
Knee flexion plateauing? Check and treat the inferior glide of the patellofemoral joint with the knee flexed!
This is such a simple and straight forward concept, but often missed in treatment. Make this small change in your practice today and see immediate gains in your patients.