D. Venusaur won't mega-evolve

I have the mega-ring and my D.Venusaur is holding the appropriate mega-stone, however the game doesn’t respond when I press ‘z’. Does anyone know if this could simply be a problem of mis-placed controls or if it’s an issue with the game/my file?
Thanks in advance!

I have just tested it out with my Typhlosion as well, it did not work

mine are working

If you’ve changed the highlighted keybind (Run), you will have to use that button instead of Z to Mega Evolve.