I had a problem with this a while back too. I read somewhere that they evolve more slowly when sheltered than outdoors so I moved mine back outside. It did seem to make a difference but it's still slow. I have seasons on a 7-day cycle so they barely had time to regrow and harvest a few times before they went dormant again so in hindsight, I would suggest leaving them sheltered until they come in season and then move them outside so you don't have to wait.
ETA: And obviously, like others have suggested, make sure they're well-tended.