Many players and coaches have come to my academy to improve consistency in a specific shot, believing the problem is technical. While it may be one of the reasons, the most of the times it is not the main one.
Although there are several reasons for not having consistency: Attention / focus, Emotional, Visual coordination, intension / responsibility and also technique. Now I want to refer to the easiest to solve: the expectation.
Suppose today you achieved your driver's license, you have 20 or 30 hours of practice, you get into your car and start driving at full speed through the center of your city, avoiding cars and people, accelerating to the maximum, taking the curves very fast and braking in the limit. Is this logical? I do not think so. The most healthy strategy would be to drive very carefully, taking all precautions and gradually taking confidence based on correct decisions and actions and in consequence, be consistent in driving.
In other words, consistency is based on the reliability that we have to perform successfully in a specific task, which gives us the confidence to, little by little, try new challenges. Then we can say that consistency is the correct balance between my actions and my abilities. Anything that goes too far of my capabilities will has very little chance of success.
"The higher the expectation the lower the consistency"
If we talk about tennis players who lack consistency, most of the times it is because they are hitting and playing in a higher speed than they really are able to manage, they have very high expectations on their shots.
To adjust this, it is necessary to understand that consistency is playing with the correct balance between technical, physical, coordination, visual and emotional capacities in relation to strategic and tactical decisions. That is, do not play too above possibilities and accept what you know/have and what you do not know/have, just be realistic: If you can not hit a topspin backhand, you can not expect to hit a short angle backhand running In an emotionally stressful situation.
"Work slow to improve faster"
This can be solved very easily, you only have to slow down until the you find the rhythm where you are able to put 15/20 balls in each rally, a status where you can feel that you are controlling the situation and not the situation controlling you.
By slowing down, you will increase the ability to organize movements and improve decision making, you will have more confidence and will magically start to hit harder and with much more consistency but with a big difference: you will now do because you choose to do. Be consistent is to decide each stroke according to the current level and not the "expected" or "desired" level. Being aware of what you are and what you have is the best starting point to design the correct strategy to improve faster.