Pierce ECL femto kit - loses effectiveness? (Nov/26/2007 )

Has any one else noticed that the Pierce ECL femto kit loses effectiveness when you get down to the bottom of the bottle(s)? It's not that the solutions are too old or expired - our lab uses a lot of it. But I've noticed that my blots don't look as good when there's only about 1-5 mls of solution left. Don't know if this is real or if I'm just imagining things.

Maybe there is a little cross contamination between the bottles?