Sickly asparagus any ideas?

I am not sure there is anything to worry about necessarily: I still get the occasional spindly asparagus like yours from some of the plants yet those same plants also give me plenty of thick, proper asparagus.

There are no other stems coming up and that one is dying.  Looks like a furtle is in order. 

Just a thought - it's not a seedling from berries on the ferns last year? Mine were all supposed to be male but I got lots of berries on the ferns which I didn't think males produced..

Just had a rummage round and there is some rotting on the crown but there are also quite a few healthy looking roots.  Not sure whether to leave them alone or remove them now. 

Tricky decision; they could well recover but it could take a couple of years to catch the others back up again but then again replacing them will involve the same wait!

I've had a few strugglers in my patch which barely managed two spindly ferns apiece but they did eventually catch up, but then again I've also lost one or two over the 6 or so years since I started with them, and probably not surprisingly it's always the younger plants.