There are many factors needs to be considered before guessing about the mileage. Few of them are:
How much amount of CNG filled-in in KGs. Every CNG dispenser has a display unit for it. If the filling amount is around 8.0 (for a 55 cylinder), then one can drive for ~120 kms local for a 1300cc engine; on a signal-less, low traffic road and gentle drive :)[max 80km/h].
If the quality of gas is not up to the mark and/or incompletely compressed (usually occurs when filling in a larger vehicle queue), still one can get low mileage.
If the gas is too hot, the cylinder may be filled in with less amount of gas, and as a result low mileage.
If any of the above point is valid, try getting CNG filled from another CNG station. If it is otherwise, get the CNG adjusted/tuned.
NOTE: It will take some time to learn how to drive on CNG to get optimum results. This might be the case.