Contact EA support, they may help you.
And mention in your message that you already posted this here, else they would probably answer just "Post it on the official forum"
I am assuming you're referring to the plants shown in the vertical lineup of 6 slots to the left of your screen? (The one containing your selected plants, with the mandatory plant at the top - Parsnip this week.)
When you attempt to use the plant food to instantly refresh any plant within that lineup, including mints, do you just tap the plant food and then tap the required plant? If that is what you are doing it will not work.
It's not like when you tap the plant food and then tap a plant that's already been placed on the playing field. You have to touch and hold the plant food and then drag it to the required plant/mint and then release it once you are sure you have the green leaf symbol on the correct plant. (It is rather easy to miss during the heat of battle and refresh the incorrect plant.)
You also have to have enough SUN for the plant to be ready when you drag over the PlantFood
Too many times I have tried to use PlantFood and did not have enough SUN
Once there is enough SUN the plant will be ready
