I have planted my gladiolus bulbs in pots. They are getting plenty of sunshine, and are being watered well. However, only a few of them have started to show any blooms, and they are not very big. In other words, the stalks have only 4-5 little flowers on them, making them look very spindly. Other stalks haven't developed any flowers at all. What makes this happen, and how can I avoid it next year? These are newly-bought bulbs.