BCIP/NBT developed blot disappearing - (Oct/09/2014 )

Hi, I've been testing a one-step BCIP/NBT solution to visualise ALP-conjugated blots. I add about 5ul of the conjugate to nitrocellulose membrane, let it dry and then add 1ml of the BCIP/NBT substrate. The dark purple spot forms after a few minutes, but here's where the problem occurs. After about an hour, the dark purple spot starts to fade, before disappearing completely. Does anyone know why this might be the case?

in what conditions are you holding your membrane? alcohol will dissolve the deposited compound.