Placebo effect is not getting stronger over time. We are getting better at controlling for all other factors: regression to the mean, self reporting related biases, the effect that people change their behaviour when they are being observed, etc.
Placebo effect is not "mind over matter" it just all the peoples behaviours, biases and life that is happening while they are getting treatment.
After doing a lot of research into homeopathy we are now very confident that it does not work and cannot work.