Posted: 08/03/2015 at 14:41
2 of mine do this, bits die back. I've always put it down to the rubbishy soil they're in.
Interestingly I have one that never does die back, it's the one in the most shaded spot The 'soil' is largely brick rubble with the odd plastic fertilizer sack thrown in.
My garden was used as a tip for years before our arrival