And fertilizing the plant isn't working either? I don't want to sound like I'd know the solution to this, though usually with the gardening skills, besides watering the plants, caring for the plants as well as communicating with these plants periodically, one thing that has been useful in helping these grow successfully has been to fertilize them with all kinds of organic plants in the inventory, and that has usually been something that has helped these plants have the "evolve" sign. Though I'm definitely going to watch to see what was the solution to this issue, since I've yet to be in the situation of TRYING to make plants evolve. That's usually quite automatic. I hope that you find the solution to that issue though! Might be a glitch in the game that hopefully they fix!