gazz ive seen this happen before if you put birds togethere then get a cold snap as we did around christmas and first few days in the new year it knocks the birds right back this happened to me 2years ago and some pairs did not lay till end of feb and march and then only after i split them again for one week and gave them plenty hemp seed they went down two weeks later that why i now have heating in my stock loft makes a big difference if you can keep the temp up early on when its cold just till they lay m8 hope this helps