platinum shiny code imports but nothing changes in game

Also make sure the code matches your exact version and region. A US Platinum code on a European ROM will often load without complaint and then just do nothing.

Most of those shiny codes are button activated, so having it enabled isn't enough, you have to hold the combo listed with the code (often something like L+R) while the battle loads. They also only touch wild encounters, not anything already in your party or boxes. If it still does nothing, check the code is for your region. A US code on a European cart just silently fails.