Because they haven't been pollenated - if they are in a greenhouse or polytunnel get a child's paint brush and transfer the pollen manually or, gently, shake is plant at least once a day to encourage the pollen to disperse. Outdoors you can still use the paint brush. If you've had the cold unseasonable weather we've had in Moray it could help.

similar answer on the second post of the question. same answer too ;o)