Hi webdesign309 – sorry for the delay.
Recurring event instances share the same post ID, so explicitly setting it (you’re passing in $post->ID as the first parameter of tribe_get_start_date()) has the effect of telling The Events Calendar to look at the first or ‘base’ event in the sequence.
However, since it looks like you’re correctly setting things up with setup_postdata(), can you pass null as the first parameter? Then The Events Calendar can magically determine which instance you are referring to 🙂
Hope that helps!