Ben Wa Balls help?

I read that when you start wearing them, you shouldn't do anything strenuous or wear them outside because even sneezing may make them drop out! Well, I've only started but I feel I can hold them in easily. But I asked my boyfriend if he could feel any difference when I consciously tightened my vaginal muscles and he said it wasn't much different from if I didn't flex at all. If I continue to use these balls, will it still make my muscles stronger even if I'm not making a huge effort to hold them in? The ones I have now are plastic. Should I get heavier ones?

placeholder text for bug in Chrome