Fully-armored doesn't mean invincible, and weapons evolved right alongside armor. See previous discussions here or here or here (all from the FAQs, credit to u/WARitter and u/MI13's answers specifically) on what plate armor could do and what weapons developed around them. And there's always the fact that if a knight in plate could be brought down to the ground (say, if he were shot by a longbow arrow, or if someone clubbed him over the head from behind) someone could slip a rondel dagger through his eyeslits or the gaps in his armor.