I have been injecting in my butt for years. I never twist around. I do it completely blind. I just use my arm and jab it in there. Practice the motion without the needle so that you figure out which part of your cheek you will hit 
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Hi @Ali4, I just noticed your question got lost in the discussion. My son is on MDI, and he is very skinny. We often do the same thing as you do - we have not noticed any effect - so I don’t think that it is likely the cause.
Sometimes I pinch and sometimes I don’t. I doubt that’s a factor.