One problem - these are all-female. Does that still apply?
I have had another think on this one and I will stick with my poor pollination theory.
I know some pundits recommend spraying plants with a fine mist of water to assist pollination set, or damping down the greenhouse to increase humidity.
Considering we have had a particularly long dry spell of late the humidity around the plants may have been negligable.
This could mean that the pollinating pollen hasn't adhered to the flower thus affecting the pollination process.
Its only a theory, but I have to admit I have never resorted to spraying before but trhen I can't recall going so long at this time of year without rain.
Trouble is most of us grow our cucumbers & tomatoes in the same greenhouse (myself included) and I don't like my tomatoes subjected to high humidity in case it promotes blight.
So perhaps a light spray is the answer!
Oh! the trials and tribulations of gardening ::) ::)