Likely you are not clearing the "Wild Ditto Appeared!" text fast enough. You have to clear that text as quick as possible for it to work. The best way to do it is to press B when its close and quickly press A immediately after. The B press gives you a few frames to buffer the input while A is pretty precise. Takes a ton of practice. I suggest making a save state as the encounter begins and just keep doing it over and over until you get the timing down. That's how I learned.