Wavy walls/ horizontal plaster bulges
First time homeowner here with a old victorian twin (semi- detached) built around 1902.
My walls are mostly plaster ( some were replaced with drywall before I bought the home). My issue is that a couple of the upstairs plaster walls are what I'd like to describe as "wavy". You can see the horizontal outline of the lath bulging out slightly. This is both the walls in the hallway, one of which I share with my neighbor. The plaster is not cracked at all but is just unsightly, especially when light is shined on it. I do have an attic directly above and those plaster walls do not have the same issue.
The walls have been the same since I bought the home a year ago so it's not a new issue.
I haven't been able to find much info on what could be causing this issue or how to fix it.
Any ideas? I'd prefer not to replace the plaster if I don't need to.