Poor potato crop

Does anyone have any ideas about this?
In two areas of the garden my main crop Desirie potatoes have done resonably well, but in the main area where I have five rows, the crop had good green growth and I thought it would be a good crop.  Harvesting today, although the growth was good, there are no potatoes, only a few very small (size of a 10p) potatoes.
What could cause this,  (the rows are overshadowed by trees for part of the day)

Regards
Simon