bah, i figured it out. like Korean, China Airlines has alot of blackout dates for SkyMiles redemptions as per this TPG post: https://thepointsguy.com/2015/05/del...d-redemptions/

although the current Delta program rules page has nothing on blackout dates for China Airlines: https://www.delta.com/content/www/en...ram-rules.html

but that must be it.