Sour crop?

Our isa brown has been unwell.  Last weekend she was poorly & I found her crop full of liquid which seemed to be solved after turning her up to be sick. that went on a few times.   Fed her on garlic & yoghurt & she seemed to come good.  But now it has returned.  I drained her yesterday, but since then there has been nothing.  Now she will not eat yoghurt & garlic, or her usual pellets, but she does go for corn & maize quiet enthusiastically. Her breath seems to be OK, not smelling at all.
Any ideas as to what it might be? Any suggestions gratefully received.

Thanks, Joyfull.  The feed is pretty new, only a couple of weeks old, but I take your point about letting her get her own way with food.  Will try mashing the pellets with warm water.  I did try grinding some, and she went for them half-heartedly.  Perhaps water will do a better job.
Thanks again.