Downloading past purchases which are marked as downloaded but can't be located

I have about 30 apps on my various ios devices which work fine on the devices but they are marked with ! in iTunes and iTunes says it can't locate them. They are not in my iTunes library or in any of my back-ups. I have transferred purchases from all my ios devices and these ones are still missing. I have located them in the iTunes store but the Download option is greyed out, I guess because it thinks I already have them. Without these apps in my iTunes I am not receiving updates automatically. How can I redownload or recover these apps so that iTunes knows I have them?

Thanks

Joanne