I have 9 laying hens, (3 white leghorns, 2 brown leghorns, 2 blackrock, & 2 plymouth barred rock). My question is, why do they keep getting bubble eye? This is the 3rd time in the 18 months i've been keeping them. On the last 2 occasions i have got the medication from the vet, & will again. Just wondering where i'm going wrong.