This is the one body part I don't worry about. It is w/o a doubt the weakest muscle group for women (I'm not saying this as someone w/a degree in exersize physiology, just as someone who's lifted for 15+ years & learned from experience).
In any case, you might want to try bench presses w/a bar rather than dumbells. Also push ups are probably one of the most effective strenght building exersizes for the chest. In addition, using a spotter will help you increase your weights. Also some gyms have dumbells that increase in 2 1/2 lb increments, which are wonderful for women. On your last set you can try using the next dumbell up in weight doing less reps (6-8 rather than 8-10).
However, don't lose sleep over not building muscle on your chest. 15 lbs is a decent weight to lift, & a woman w/an over-muscular chest is not the most attractive thing in the world.