Tree Ferns

Over the past three years our Dicksonia antarctica has declined to such an extent that the fronds are now only, on average, 24 inches long.  We keep it watered and spray over the fronds once a week.  This year from frond growth we gave it liquid fern feed.  It is well covered during the winter months, but nothing we do seems to get it to grow as it used to.  It is approximately 1.2m in height.  The other problem, apart from poor frond growth is that the crown appears to be 'closing-up' each year - see photographs.  Any advice anyone can give would be much appreciated.