Levelling up plants

Not sure how stupid I’m being here (please bear with me as I’ve only had the app for a few weeks) but I can’t seem to find a way to upgrade my plants to stage 3. I’ve tried the nursery in the garden but it says that I can’t use it until I have at least 18 plants at stage 2 (which I do already have). Not sure what else to do as I feel I have clicked everything possible.
Would be a wonderful help! Tysm!