Why are some mature broad bean pods empty, with immature, undeveloped beans?
Broad beans need the right day-length and a temperature of about 20°C at flowering. If the beans are planted too early in the spring, & there is a blip in the weather as the first beans flowered, the beans will not set well.
Crops planted too late will also produce low yields and will be affected by high temperatures later in the season.
Broad beans produce less well in windy areas. Double rows (20 cm apart) help the plants to withstand wind. Otherwise a windbreak pays dividends.
Broad bean pods tend to be empty at the start of the season, before the conditions are right for the beans to set.