Depending on what you have growing, I would guess there are no insects around, or too much rain.
Both instances will cause 'petal drop' because they haven't been fertilized. Melons, pumpkins, Etc. are notorious for this, due to not producing any MALE flowers.If this is the case, then maybe in a month you will see some results.
Don't use insecticide during this flowering season. you'll kill all the insects needed for fertilization
Only after fruit has set
