injection sites - do they recover?

hi i noticed when i inject in my stomach insulin doesnt work sometimes and sometimes it works well i guess this could be because iv been injecting in my stomach 4x a day for the past 5years more less in same spots with not much rotations also i have stretch marks on my stomach from along time ago so not sure if this is somthing else that could affect insulin absorbtion i today injected in my bottocks area lol and it seems to be workign the insulin can old injection sites get better meaning could they be used again and could they absorb insulin as they use to if you stop injecting in them for long periods of time