Have you taken her to see specialists? I would think maybe a rheumatologist, endocrinologist, immunologist - not necessarily in that order... But I don't know who you've had see her (what type of doctor I mean) or what they've checked her for already.
sometime viruses can cause the WBC to lower...I'm saying this is the cause but it is possible. It isn't clear to me what is wrong w/ your daughter based on your description.
leukopenia can be caused by other things to...medications, other infections, etc. I'm sure your docs are the best source for that info.