Retin-A

Is it possible for the skin to become immune to Retin-A? I've been
using 0.1%, non-micro version for 3 weeks. It did improve my skin in
the first week I no longer think it's making a difference. It also
doesn't bother my skin either. I tried using it twice per day for a
few days, and that didn't affect my skin as in no change or adverse
side effects. Is there anything stronger then 0.1%? Is this normal
not to feel any effect this short in the game? Or does it continue to
"work" as long as you use it?