We have had 2 tree ferns in the garden for the last 4 years and have had no problems. This year the one above has made new fronds which have started to rise but have not developed further. Instead of unfurling they seem to have dried apart from the one you can see which has remained like this. The other tree fern in the same border is fine and both have had the same care, feeding etc. Does anyone have any ideas what may have caused this to happen?