[phenixbb] difference density maps

Hi, I have a question on difference density maps. We are soaking in a substrate into my enzyme and am trying to see if it is in there by looking at difference density maps. We looked at an fobs-(fobs-fcalc) map and see density and 5 sigma at each catalytic active site in roughly the same place, its not great, but it seems like there is something there. so then, I thought I would confirm this by an fobs-fobs map. But I just used the phenix fobs-fobs difference electron density map and I dont see anything there. So I am curious as to an explanation of the difference. Any ideas? Raj