If you live in a humid climate, even CP and HP soap will sweat wherever it is (talking through experience here), and I guess more so for M&P, and more in the shower with all the humidity in there.....
Its the nature of lye based soap, compared to syndets. Only way around it is to find a solution to the humidity (hard to do in any room, much more in a shower lol) by blowing a fan on them, keeping an AC on in the room for a certain amount of time during the day, or using a dehumidifier.
I, unfortunately, have to do all three. Sigh..