Experts and BHG readers answer.
bell peppers falling off
Your peppers may not be getting properly pollinated. While they can self pollinate, it may help to gently shake the plants/flowers while they are in bloom to disperse the pollen within the flower. Peppers are also temperature sensitive. They don't set fruit well when temperatures are either too low or too high. (Some varieties are less sensitive than others, so if this seems to be a likely cause, you may want to try other varieties that are better adapted to the heat, for instance.)