A couple of notes that you may want to check on. The counter will hit an airborne opponent. Check your inputs (214 instead of 236). The extend action is holding down the button. Take them away from the corner, maybe? (worked for me). Try countering them at different heights (maybe closer to the ground).
At first I too, was wondering why it wasn't working, but I kept trying and within a few more tries it popped. Sorry if this isn't much help, but I didn't have to make that many attempts to figure out what helps.