I'm gonna let you all have this insight for free.
Many times, women don't know how to receive a compliment especially when they see how flawed they are. I struggled with this when dealing with how God sees me. My flaws are ever before me reminding me of how unworthy and imperfect I am, but yet God sees the beauty of who he made me to be and who I am becoming in Him.
The other thing, is that many women don't trust compliments. In other words, they are unsure if you're saying it because you really honestly believe it, or if there's another agenda or motive. There's a lot that plays into why some feel that way.
Then there's the never ending comparison to what is beautiful in our eyes, or what the majority of society deems beautiful. A lot of us don't measure up to that, so when you pay a compliment, the thought is, you just haven't seen what's out there, lol.
Women are under constant pressure about how they look, where men basically get away with a lot more. I just saw an overweight LGMer post a FB status hating on overweight women, and I was thinking to myself, "You got a whole heck of a lot of nerve!" LOL! Anyway, those are just some ideas from my own personal experience and observations of many women that are close to me.
Oh yeah, and another final reason is fear of being viewed as conceited or whatever. Some were never taught how to receive a compliment and celebrate themselves. You can still be humble and do that...