Okay so I think it was definitely a lack of nutrients. The smaller plant hadn't shown any sort of real growth for a while, but I gave it some nutes and now it's already got two new leaves and a set starting to grow. Same goes for the bigger plant.
Little update; I just transplanted them both to a bit larger pots, I think they're half gallon pots. Unfortunately the only soil I had was miracle grow. I thought it was really interesting to see how much goes on underneath the surface. I didn't realize they had such extensive root systems already.