Flowers dropping off can be caused by a number of things. Over watering or under watering can cause it, but also a lack of bees. They are needed to pollinate, which makes the flowers become fruit.
Do you have bees around your yard? If not, you can do things to attract them such as plant things they like. I'm not sure what climate zone you are in, so can't really give suggestions there. There are also bee attractants available. In the meantime, you can also try and hand pollinate. Take a small artist paint brusch and rub each of the flowers.