Some of mine have only evolved once even with a gardener there daily they've been through my whole sims life and never gone past nice some of them. The lot has the good soil trait I thought that would help but it's not.
Actually I haven't had a problem with evolving plants. You just go to find the right fertilizer. If you can find grow fruit and plant it, I believe it is available in OD retweet to buy. That is the best fertilizer and its special effect allows to plant to go to one level. I hope the update didn't change it, but I haven't tried it after the update.
Since you said this I've been trying the bees. It's been almost 30 sim days and my roses which get bee action are still stuck about 10 pixels shy of leveling up from excellent. Most of the plants are not budging at all, I had one plant evolve since the bees and they pretty much go several times a day. I think this is a real bug I don't know why some of us have it and some don't they do not improve at all.
Mine are sheltered if that makes a difference.
