Im still to master the compliment bit. Even when I say "that looks great" I still get "you don't like it do you, what's wrong with it?" Or "do you notice anything different" I say "yeah that's a nice top" but then she says "ive had that for ages, I was talking about my hair!"
So either your compliments are exaggerated or dishonest and she can sense that, or she is very preoccupied with her looks and very self-critical.
As for the latter, it's all about women wanting their guys to pay 200% attention to every detail of her. That's an absolutely unrealistic demand.
Maybe it'd help to admit that you don't always pay full attention to her hair or her shirts or don't care much for whether stuff she's wearing looks great or just okay, because those things aren't as important to you as they are to her. Because you always find her beautifull inside and outside, hairstyling and clothing are thus none of your concerns, and you like her best when she's naked anyway ;)... or something along those lines.
A boyfriend I had long ago did often ask me for my opinion on his appearance, especially when wearing something new or after having had his hair cut. He was a bit insecure in those days. I always told him that he looks hot. Because he just always looked hot. His whole being - his looks, his intelligence, his personality, his soul - was beautifull and attractive. But after a while he started complaining that my opinion is meaningless because it's always the same and there's no variance in it. So basically in his eyes I was like a clock that is stuck and always showing the same time, so you can't get any real information about the time from it. That was a bit saddening. :-(
So, yeah, men can be as complicated as women about compliments.
It's all about insecurity I guess.