streptavidin losing magnetic properties?

[jhalpin]

Anyone have any experience with streptavidin beads losing their magnetic properties? They have not been boiled nor frozen. It's for an antibody binding assay and it's really random as to which wells this happens in. We think it may be from the particular samples added but would like to identify what component in particular could cause this. My best guess at this point is something modifying the iron molecule in the bead?

Thanks!