The Slider Name is what connects the Slides to the Flex Container. Both should have the same Slider Name.
By default the Flex Container and Slides both have a Slider Name of MediaSlider. If you choose to add additional slides, make sure that the Slider Name matches.
If you want to change the Slider Name from the default because you have more than one Flex Slider, do not use any spaces or special characters for the gallery name.
Find the Slider Name for the Flex Container by clicking on Flex, opening the options, and open SLIDER NAME.
Find the Slider Name for SLIDES by clicking on the slides, opening the options, and open MAIN CONTROLS.