Is your battery charged to 100%? You can check this in the app, or in the MMI system under Car > Charging & efficiency > Charging
Like any other car, how you drive will drastically affect what range you get out of it. Just as thrashing a conventional combustion engine car will result in terrible MPG, hard acceleration, not using regeneration, and driving at consistently high speeds will affect the estimated range.
You should be able to achieve 2.5 miles per kilowatt/hour in the E-Tron. That equates to just over 160 miles of range in the 50, and 210 in the 55. However, if you're driving hard, you'll see that drop, and the software will calculate your range a lot lower.
Check the efficiency you've achieved recently under "Driving Data" in the app. That will tell you whether the range you're seeing is correct, or if you have a fault.
I like to keep an eye on the efficiency level I am achieving when I drive. You can add this information to your virtual cockpit by going to Settings > Display & brightness > Virtual Cockpit and enabling the "Electrical consumption" display. This will show you your current miles/kWh, and your trip average.