Ionian Boots + Insight Mastery

Insight Mastery - Reduces Summoner Spell cooldown by 15%.

Ionian Boots - Reduces Summoner Spell cooldown by 10%.

The Insight Mastery worked just fine when I was playing a game as Kassadin; Flash cooldown was at 255s after use. However, after buying Ionian Boots and using Flash, the cooldown still started at 255s.