Does she want to evolve, anonymous? Sometimes, pokémon resist evolution on their own out of their own personal preference. (Some do this better than others, which is why an everstone isn’t always needed.) If she seems to be under a lot of stress, that could be it. It’s important to talk to her, then, to determine her wants and needs, and if she does indeed want to remain a bayleef, get her an everstone to help her.
On that note, it’s true that her environment could also factor into an actual inability to evolve. Evolution takes a lot of energy, so it’s easiest for grass-types like your bayleef to evolve in spring or summer. Alternatively, pollution and stress also stunt evolution, so if you live in the city, consider a trip out to the country. You’ll need at least a week of relaxation before your bayleef would be ready to evolve.
Best of luck, anonymous! (…although as a tangent, it always confuses me that some trainers refer to battling prowess and experience with levels. Maybe I’ve lost touch with the trainer system, but I will never make heads or tails of that…)