Does anyone know what is happening with this plant...

Some plants are just a bit slower than others or maybe all the rest are faster than this one, and she is the normal one! Being so deep in the pot also means she is missing out on a lot of reflected light, which could also be contributing to her slower growth. In reality, you should fill your pots right up, as the extra room for the roots to grow will provide for much more potential above ground growth. It is a bit like a swimming pool, no point in only half filling it, otherwise you should have bought a smaller swimming pool instead. Besides, the substrate is usually the cheapest component in the big picture and no point in skimping on it if you are after maximum returns for your efforts. Hope this helps,.... Organoman.