I'm always curious about this. Few days ago, my friend was complaining that she needs to change her skincares since it's no longer working on her skin. It made me curious. Can skincare actually stop working? Can our skin becoma immunes to the skincare we use? Is that possible?
I'm not a beauty scientist but it is more likely that your skin's condition has changed. I find sometimes when I ADD a new product to my regimen that it can cause changes.
Mostly, however, I think we get bored and want to try something different.
I'm not aware of any scientific study to show that skin care stops working, but Judy is right, your skin can change which could affect how your skin care product works.
My skin does the best when I stick to the same products. I have to switch to a heavier moisturizer in the fall and then back to a lighter one on the summer and the switch always triggers a minor breakout. However, I have dry skin so I'm not using any acne treatments or products for oily, acne-prone skin. I've heard that many acne treatments will work for 3 to 6 months and then stop being effective. I know someone who rotates her products every 2 or 3 months because of this. She claims if she changes them before they stop working that she can keep her skin clear all the time.