If you are driving long distances and/or lats of stop-n-go in traffic, you would be running mostly beyond the electricity reserve on gas and 30 mpg could happen (it has dropped that low for me). But if you can runabout under about 20 miles, you would be running almost completely on battery.
If neither of these apply, I wonder if your batteries are worn out...
The c-max is not known for good gas mileage as compared to other hybrids. Part of the reason is the extra 600lbs of batteries needed for the all-electric driving. ~30 is a bit low unless as mentioned above, it's winter and you're using heat and doing more stop-and-go driving. I'd say high 30's is typical in good driving conditions.