While this is most typically a function of UA's need to verify your documents and thus accomplished quickly, it can also be due to your reservation being out of "synch" a common UA IT fault.
It is well worth calling UA. If it's the likely former issue, just show up early enough to sustain a line at check-in. If it's the latter, a phone agent can fix it for you on the spot.
If it's something else and problematic, all better to fix it before the airport.