Are you finding inventory for your entire one-way journey (all segs) and requesting such, or just finding inventory for one segment of your trip?

If you find inventory for one segment (but not the other) and are requesting an upgrade for just that segment, I'd go ahead and ask them to waitlist you for that specific segment even if they say there is no inventory. I've always had it clear shortly after waitlisting, even when the agents seemed to be (incorrectly) telling me that there was nothing available.

There is something weird to do with upgrading the whole trip vs just one segment, feel free to read the big RUC/GUC thread for more info.