Lauren. You are well on your way to becoming a plant expert. The best teacher of all is "doing". Believe me, I have killed more plants than you would believe (I've been growing tropical plants [and others] for over 40 years). I learn a lesson every time I have had a failure, so losing plants, or having them simply grow poorly, is part of the learning-curve.
One important factor is light. Some plants can get by with little but most plants grow best in bright light. The leggy or poorly growing plants are most likely due to too little light or not enough hours of it. What fluorescent fixture are you using?