The official policy is that there is not different inventory for DSU availability for paid versus P+C rates. Hyatt has addressed this on FT before (for example, here: http://www.flyertalk.com/forum/23121726-post5.html).

But the computer system will sometimes say there's no inventory, and so I think a bunch of HGP staff (phone reps, even Twitter folks) have mistakenly come to believe there's different buckets of inventory. Very frustrating that they don't do better training about their own official policy.

In my experience, sometimes it requires HGP to work with the individual hotel's reservation system to get the DSU to go through with a P+C rate.