I just ran into this problem and figured out what is wrong. The navigation buttons have the same z-index as the content in the slider. Once I changed both of the navigivation buttons to have a z-index of 11 instead of 10, they worked again.
You need to manually modify the content-slider.php file of this plugin.
This is an example of what my css looks like for the plugin:
#featured_slider .feat_prev {
background: transparent url(<?php echo $direct_path;?>/images/sprite.png) no-repeat;
background-position: 0px 0px;
width: 17px;
z-index: 11;
height: 16px;
position: absolute;
left: 20px;
cursor: pointer;
bottom: 15px;
float: left;
}
#featured_slider .feat_next {
background: transparent url(<?php echo $direct_path;?>/images/sprite.png) no-repeat;
background-position: -17px 0px;
width: 17px;
z-index: 11;
height: 16px;
position: absolute;
left: 40px;
bottom: 15px;
cursor: pointer;
}
notice the z-index value.