Hi Melissa, you would need to develop your own logic to remember a slide has been completed and enable the Next button if it has. For example, you could have a variable for the slide, that is set to true when the Next button is enabled. Then when the slide is revisited, use the variable to determine if the Next button should be enabled/disabled.
Also, double check the slide properties, and check to see if you have "Resume saved state" selected.
Note: Be aware though that using "Resume saved state" on a lot of slides can max out a field called suspend_data on the LMS if running as SCORM 1.2. This can cause some unpredictable tracking behaviour.