Others have mentioned this but it if you haven't stripped them, do. I scoured the internet to look for info about my specific diapers (the official websites often have instructions or approved detergent) and it worked out well for me.
Might clear it right up. If it doesn't then you know your diapers are as close to new as they can be :)
I also bought disposible liners (sounds crazy, or did to me) but can be a big help at night, esp. if you are using creams and ointments. Gerber and Kushies sell them I know, probably other brands out there too.
I went the route of disposibles at night when I started having this problem until I stripped and the combo of a break from cloth and cleaning the diapers of any built up stuff cleared things up for us.