1st timer on this forum but old timer at growing flowers. Yellow bells is suppose to do well in a container. I've tried 3 times without success. I am using Franklin potting soil in a 3 cu ft fiberglass pot. Full sun and tried different water and fertilizing amounts each time. Lower leaf tips start turning brown and it progresses upwards. In the end all the mature leaves are brown and the new shoots that come out and soon wilt until there are no leaves left. What am I doing wrong?