Do you know if there are plenty of insects (bee's etc) in your garden? If your plants aren't getting pollinated then the flowers will drop. You could always try doing it yourself with a cotton bud or very fine artists paint brush.
Use your weapon of choice to go between the flowers and very, very gently brush pollen from one flower to the next. Keep to a fairly small area at a time.

Another trick is to water your plants all over with a fine spray, like rain as this can aid pollination!

Sparrows - they love 'em, eat bits and the rest fall, so if you have sparrows about, that'll be the answer probably. Otherwise, they're falling because they're too dry at the roots (this is one of the reasons for digging a trench and filling with humus rich materials and planting into that - it retains water better).
Having said that, the point about bees is also valid - there do seem to be fewer of them about this year, judging by the lack of pollinated flowers on my pelargoniums this year - I've only seen one single capsule.

Today I noticed that the red flowers on our runners are setting for the first time this year, even though the plants have reached the top of the canes.
This is because the weather has suddenly turned from dry and extremely hot to very humid and much cooler.
Runners come from cool mountainsides in Central America.