Historically United Express flights have had problems posting as occasionally OZ's system sees only the operating carrier (ie SkyWest or ExpressJet) rather than the flight operating as a UA flight. Particularly in 2011-2012 there were only a small bank of UAX flights that could be credited, all others were rejected.

I have only flown UA mainline recently, so I am not sure if this has been fixed or not, even in 2012 it looked like the there was a fix in place. Although the effectiveness of the fix was somewhat spotty.

I would second jon503's recommendation, give it a bit of time, say a few weeks for the credit to appear, if not try a retrocredit.

If a retrocredit does not work, I doubt you will end up getting credit by pushing at a call centre. The OZ human robots are not known to handle such issues very well.

For interest:
United Express Flights Not Eligible OZ Credit