Lily, when you say you only use top soil do you mean garden soil, or is it what some stores sell as "top soil" which is usually not soil at all. Garden soil doesn't work well in pots so it would be useful to know what it actually is. The new leaves look a little distorted to me so I'm not sure it is a nutrient issue. It does look as though there are long stems behind the zinnia leaves, not sure if it is the zinnias that are leggy and floppy? If they are then at least part of the problem may be not enough light. Are they growing in a house, greenhouse or outdoors?
