I agree with classic, I used the rebuilt kit from pinball resource and my chimes sound good now. If I had to guess the rubber rings you used are too thick and aren't providing enough room for the bars to move and ring.

Just keep in mind Williams chimes are inferior in design to gottlieb and bally chimes. Even rebuilt, Williams chimes are not as loud and sound clanky no matter what you do. Personally I find it charming and adds to the personality, but you'll never get anything sounding as good as Gottlieb's chimes from a Williams chimebox.