I had the same problem with the rubber ear piece popping out. It was made doubly bothersome because the silicon jawbone tip would not make contact with my cheek making the noise-cancellation work against me.
To fix the problem i had to take two steps.
1. follow the advise above and rotate the earpiece from top to bottom to put it on.
2. Try some of the other rubber ear pieces on. I found the semi-triangular one the best fit. It stays in my ear without any trouble.
I wouldn't trade this bluetooth set for any other!!