Everytime my greninja does mat block, it keeps on failing! After doing mat block [ I'm doing double battles ] I get my charizard to do heat wave! Whats wrong?
>Mat Block will only work if used on the first turn after the Pokémon enters the field.
This means you can only use it once per switch in. You'll have to switch out Greninja if you want to use Mat Block again.
Source: Bulbapedia