Several issues were present with missing data, notably with none being returned by Bridgend, Merthyr Tydfil and Wrexham councils.
Roedd sawl mater yn ymwneud â data coll, yn benodol ni chafodd unrhyw ddata ei ddychwelyd gan gynghorau Pen-y-bont ar Ogwr, Merthyr Tudful a Wrecsam.