Flexslider image height won't adjust

I am using the Recent Posts Flexslider plugin for a Wordpress site. The container is working fine in that it resizes for different screens, but when I scale down to smaller screen sizes the image height doesn't adjust. This leaves me with white space. I have

img{height:auto;} 

and

.flexslider .slides img{
width: auto; height: auto; max-width: 100%; max-height: 100%; margin: 0 auto; vertical-align: middle;

}

in my css. Can anyone tell me what I am doing wrong?