Wilting asparagus problem?

I planted 50 ginjlim asparagus crowns in my allotment 2 years ago. Last year I took a small crop and this year I have taken more. I have noticed that some of the spears, maybe 10% have died off shortly after appearing. These bad spears seem to be pale in appearance, bent over and have a kind of wilted appearance. Anyone know what could be causing this? I can only think its either the cold weather (possibly frost?) or maybe slugs (I have put pellets and wood chip mulch around). All info welcome.