Lots of reasons, lack of water, not ridged up properly, main crop potato and not ready to harvest yet... alternatively 1st early that does not produce large potatoes. Join GoY Guest and then you can tell us more.

Potatoes normally needs a reasonable quantities of phosphate in the form of Calcim superphosphate available in 'Wilkinson's garden centre' in absence of which potatoes does not get maturity. Another factor is if there is too much nitrogen fertilizer, then plant give enormous leaves. If the place is devoid of plenty of sunshine,the same things also happen. If every condition is fulfilled, there is no chance for the poor yield of potatoes.
Thank you very much for the question and waiting for your reply.
BARI

Bari Guest can't reply to you and I've never used calcium superphosphate yo help grow potatoes.